Ancient methods.
The most elementary type of fortification is the thorn _hedge_, a type
which naturally recurs from age to age under primitive conditions. Thus,
Alexander found the villages of the Hyrcanians defended by thick hedges,
and the same arrangements may be seen to-day among the least civilized
tribes of Africa. The next advance from the hedge is the _bank_ of
earth, with the exterior made steep by revetments of sods or
hurdle-work. This has a double advantage over the hedge, as, besides
being a better obstacle against assault, it gives the defenders an
advantage of position in a hand-to-hand fight. Such banks formed the
defences of the German towns in Caesar's time, and they were constructed
with a high degree of skill. Timber being plentiful, the parapets were
built of alternate layers of stones, earth and tree trunks. The latter
were built in at right angles to the length of the parapet, and were
thus very difficult to displace, while the earth prevented their being
set on fire. The bank was often strengthened by a palisade of tree
trunks or hurdle-work.
After the bank the most important step in advance for a nation
progressing in the arts was the _wall_, of masonry, sun-dried brick or
mud. The history of the development of the wall and of the methods of
attacking it is the history of fortification for several thousand years.
The first necessity for the wall was height, to give security against
escalade. The second-was thickness, so that the defenders might have a
platform on the top which would give them space to circulate freely and
to use their weapons. A lofty wall, thick enough at the top for purposes
of defence, would be very expensive if built of solid masonry; therefore
the plan was early introduced of building two walls with a filling of
earth or rubble between them. The face of the outer wall would be
carried up a few feet above the platform, and crenellated to give
protection against arrows and other projectiles.
The next forward step for the defence was the construction of _towers_
at intervals along the wall. These provided flanking fire along the
front; they also afforded refuges for the garrison in case of a
successful escalade, and from them the platform could be enfiladed.
The evolution of the wall with towers was simple. The main requirements
were despotic power and unlimited labour. Thus the finest examples of
the system known to history are also amongst the earliest. One of these
was Nineveh, built more than 2000 years B.C. The object of its huge
perimeter, more than 50 m., has been mentioned. The wall was 120 ft.
high and 30 ft. thick; and there were 1500 towers.
After this no practical advance in the art of fortification was made for
a very long time, from a constructional point of view. Many centuries
indeed elapsed before the inventive genius of man evolved engines and
methods of attack fit to cope with such colossal obstacles.
The earliest form of attack was of course _escalade_, either by ladders
or by heaping up a ramp of faggots or other portable materials. When the
increasing height of walls made escalade too difficult, other means of
attack had to be invented. Probably the first of these were the _ram_,
for battering down the walls, and _mining_. The latter might have two
objects: (a) to drive an underground gallery below the wall from the
besiegers' position into the fortress, or (b) to destroy the wall itself
by undermining.
The use of missile _engines_ for throwing heavy projectiles probably
came later. They are mentioned in the preparations made for the defence
of Jerusalem against the Philistines in the 8th century B.C. They are
not mentioned in connexion with the siege of Troy. At the sieges of Tyre
and Jerusalem by Nebuchadnezzar in 587 B.C. we first find mention of the
ram and of movable towers placed on mounds to overlook the walls.
Classical times.
The Asiatics, however, had not the qualities of mind necessary for a
systematic development of siegecraft, and it was left for the Greeks
practically to create this science. Taking it up in the 5th century B.C.
they soon, under Philip of Macedon and Alexander, arrived at a very high
degree of skill. They invented and systematized methods which were
afterwards perfected by the Romans. Alexander's siegecraft was extremely
practical. His successors endeavoured to improve on it by increasing the
size of their missile and other engines, which, however, were so
cumbrous that they were of little use. When the Romans a little later
took up the science they returned to the practical methods of Alexander,
and by the time of Caesar's wars had become past-masters of it. The
highest development of siegecraft before the use of gunpowder was
probably attained in the early days of the Roman empire. The beginning
of the Christian era is therefore a suitable period at which to take a
survey of the arts of fortification and siegecraft as practised by the
ancients.
Conditions at opening of the Christian era.
In fortification the wall with towers was still the leading idea. The
towers were preferred circular in plan, as this form offered the best
resistance to the ram. The wall was usually reinforced by a ditch,
which had three advantages: it increased the height of the obstacle,
made the bringing up of the engines of attack more difficult, and
supplied material for the filling of the wall. In special cases, as at
Jerusalem and Rhodes, the enclosure walls were doubled and trebled.
Citadels were also built on a large scale.
The typical site preferred by the Romans for a fortified town was on
high ground sloping to a river on one side and with steep slopes
falling away on the other three sides. At the highest point was a
castle serving as citadel. The town enclosure was designed in
accordance with the character of the surrounding country. Where the
enemy's approach was easiest, the walls were higher, flanking towers
stronger and ditches wider and deeper. Some of the towers were made
high for look-out posts. If there was a bridge over the river, it was
defended by a bridge-head on the far side; and stockades defended by
towers were built out from either bank above and below the bridge,
between which chains or booms could be stretched to bar the passage.
The natural features of the ground were skilfully utilized. Thus when
a large town was spread over an irregular site broken by hills, the
enceinte wall would be carried over the top of the hills; and in the
intervening valleys the wall would not only be made stronger, but
would be somewhat drawn back to allow of a flanking defence from the
hill tops on either side. The walls would consist of two strong
masonry faces, 20 ft. apart, the space between filled with earth and
stones. Usually when the lie of the ground was favourable, the outside
of the wall would be much higher than the inside, the parapet walk
perhaps being but a little above the level of the town. Palisades were
used to strengthen the ditches, especially before the gates.
There was little scope, however, in masonry for the genius of Roman
warfare, which had a better opportunity in the active work of attack
and defence. For siegecraft the Roman legions were specially apt. No
modern engineer, civil or military, accustomed to rely on machinery,
steam and hydraulic apparatus, could hope to emulate the feats of the
legionaries. In earthworks they excelled; and in such work as building
and moving about colossal wooden towers under war conditions, they
accomplished things at which nowadays we can only wonder.
The attack was carried on mainly by the use of "engines," under which
head were included all mechanical means of attack--towers, missile
engines such as catapults and balistae, rams of different kinds,
"tortoises" (see below), &c. Mining, too, was freely resorted to, also
approach trenches, the use of which had been introduced by the Greeks.
The object of mining, as has been said, might be the driving of a
gallery under the wall into the interior of the place, or the
destruction of the wall. The latter was effected by excavating large
chambers under the foundations. These were supported while the
excavation was proceeding by timber struts and planking. When the
chambers were large enough the timber supports were burnt and the wall
collapsed. The besieged replied to the mining attack by countermines.
With these they would undermine and destroy the besiegers' galleries,
or would break into them and drive out the workers, either by force of
arms or by filling the galleries with smoke.
Breaches in the wall were made by rams. These were of two kinds. For
dislodging the cemented masonry of the face of the wall, steel-pointed
heads were used; when this was done, another head, shaped like a ram's
head, was substituted for battering down the filling of the wall.
For escalade they used ladders fixed on wheeled platforms; but the
most important means of attack against a high wall were the movable
towers of wood. These were built so high that from their tops the
parapet walk of the wall could be swept with arrows and stones; and
drawbridges were let down from them, by which a storming party could
reach the top of the wall. The height of the towers was from 70 to 150
ft. They were moved on wheels of solid oak or elm, 6 to 12 ft. in
diameter and 3 to 4 ft. thick. The ground floor contained one or two
rams. The upper floors, of which there might be as many as fifteen,
were furnished with missile engines of a smaller kind. The archers
occupied the top floor. There also were placed reservoirs of water to
extinguish fire. These were filled by force pumps and fitted with hose
made of the intestines of cattle. Drawbridges, either hanging or
worked on rollers, were placed at the proper height to give access to
the top of the wall, or to a breach, as might be required. Apollodorus
proposed to place a couple of rams in the upper part of the tower to
destroy the crenellations of the wall.
The siege towers had of course to be very solidly built of strong
timbers to resist the heavy stones thrown by the engines of the
defence. They were protected against fire by screens of osiers,
plaited rope or raw hides. Sometimes it was necessary, in order to
gain greater height, to place them on high terraces of earth. In that
case they would be built on the site. At the siege of Marseilles,
described by Caesar, special methods of attack had to be employed on
account of the strength of the engines used by the besieged and their
frequent sallies to destroy the siege works. A square fort, with brick
walls 30 ft. long and 5 ft. thick, was built in front of one of the
towers of the town to resist sorties. This fort was subsequently
raised to a height of six storeys, under shelter of a roof which
projected beyond the walls, and from the eaves of which hung heavy
mats made of ships' cables. The mats protected the men working at the
walls, and as these were built up the roof was gradually raised by the
use of endless screws. The roof was made of heavy beams and planks,
over which were laid bricks and clay, and the whole was covered with
mats and hides to prevent the bricks from being dislodged. This
structure was completed without the loss of a man, and could only have
been built by the Romans, whose soldiers were all skilled workmen.
Although these towers were provided with bridges by which storming
parties could reach the top of the wall, their main object was usually
to dominate the defence and keep down the fire from the walls and
towers. Under this protection breaching operations could be carried
on. The approaches to the wall were usually made under shelter of
galleries of timber or hurdle-work, which were placed on wheels and
moved into position as required. When the wall was reached, a shelter
of stronger construction, known as a "rat," was placed in position
against it. Under this a ram was swung or worked on rollers; or the
rat might be used as a shelter for miners or for workmen cutting away
the face of the wall. The great rat at Marseilles, which extended from
the tower already described to the base of the tower of the city, was
60 ft. long, and built largely of great beams 2 ft. square, connected
by iron pins and bands. It was unusually narrow, the ground sills of
the side walls being only 4 ft. apart. This was no doubt in order to
keep down the weight of the structure, which, massive as it was, had
to be movable. The sloping roof and sides of timber were protected,
like those of the tower, with bricks and moist clay, hides and wool
mattresses. Huge stones and barrels of blazing pitch were thrown from
the wall upon this rat without effect, and under its cover the
soldiers loosened and removed the foundations of the tower until it
fell down.
In order that it might be possible to move these heavy structures, it
was usually necessary to fill up the ditch or to level the surface of
the ground. For this purpose an "approach tortoise" was often used.
This was a shelter, something between the ordinary gallery and the
rat, which was moved end on towards the wall, and had an open front
with a hood, under cover of which the earth brought up for filling the
ditch was distributed.
The missile engines threw stones up to 600 lb. weight, heavy darts
from 6 to 12 ft. long, and Greek fire. Archimedes at the siege of
Syracuse even made some throwing 1800 lb. The ranges varied, according
to the machine and the weight thrown, up to 600 yds. for direct fire
and 1000 yds. for curved fire. At the siege of Jerusalem Titus
employed three hundred catapults of different sizes and forty
balistae, of which the smallest threw missiles of 75 lb. weight. At
Carthage Scipio found 120 large and 281 medium catapults, 23 large and
52 small balistae, and a great number of scorpions and other small
missile engines.
Screens and mantlets for the protection of the engine-workers were
used in great variety.
In addition to the above, great mechanical skill was shown in the
construction of many kinds of machines for occasional purposes. A kind
of jib crane of great height on a movable platform was used to hoist a
cage containing fifteen or twenty men on to the wall. A long spar with
a steel claw at the end, swung in the middle from a lofty frame,
served to pull down the upper parts of parapets and overhanging
galleries. The defenders on their side were not slow in replying with
similar devices. Fenders were let down from the wall to soften the
blow of the ram, or the ram heads were caught and held by cranes.
Grapnels were lowered from cranes to seize the rats and overturn them.
Archimedes used the same idea in the defence of Syracuse for lifting
and sinking the Roman galleys. Wooden towers were built on the walls
to overtop the towers of the besiegers. Many devices for throwing fire
were employed. The tradition that Archimedes burnt the Roman fleet, or
a portion of it, at Syracuse, by focusing the rays of the sun with
reflectors, is supported by an experiment made by Buffon in 1747. With
a reflector having a surface of 50 sq. ft., made up of 168 small
mirrors each 6 by 8 in., lead was melted at a distance of 140 ft. and
wood was set on fire at 160 ft.
The development of masonry in permanent fortification had long since
reached its practical limit, and was no longer proof against the
destructive methods that had been evolved. The extemporized defences
were, as is always the case, worn down by a resolute besieger, and the
attack was stronger than the defence.
Middle ages.
Through the dark ages the Eastern Empire kept alive the twin sciences of
fortification and siegecraft long enough for the Crusaders to learn from
them what had been lost in the West. Byzantium, however, always a
storehouse of military science, while conserving a knowledge of the
ancient methods and the great missile engines, contributed no new ideas
to fortification, so far as we know. In practice the Byzantines favoured
multiplied enceintes or several concentric lines of defence. This of
course is always a tendency of decadent nations.
In the West the Roman fortifications remained standing, and the
Visigoths, allies of Rome, utilized their principles in the defences of
Carcassonne, Toulouse, &c. in the 5th century. Viollet-le-Duc's
description and illustrations of the defences of Carcassonne will give a
very good idea of the methods then in use:--
"The Visigoth fortification of the city of Carcassonne, which is still
preserved, offers an analogous arrangement recalling those described
by Vegetius. The level of the town is much more elevated than the
ground outside, and almost as high as the parapet walks. The curtain
walls, of great thickness, are composed of two faces of small cubical
masonry alternating with courses of brick; the middle portion being
filled, not with earth but with rubble run with lime. The towers were
raised above these curtains, and their communication with the latter
might be cut off, so as to make of each tower a small independent
fort; externally these towers are cylindrical, and on the side of the
town square; they rest, also towards the country, upon a cubical base
or foundation. We subjoin (fig. 1) the plan of one of these towers
with the curtains adjoining. A is the plan of the ground-level; B the
plan of the first storey at the level of the parapet. We see, at C and
D, the two excavations formed in front of the gates of the tower to
intercept, when the drawbridges were raised, all communication between
the town or the parapet walk and the several storeys of the tower.
From the first storey access was had to the upper crenellated or
battlemented portion of the tower by a ladder of wood placed
interiorly against the side of the flat wall. The external
ground-level was much lower than that of the tower, and also beneath
the ground-level of the town, from which it was reached by a
descending flight of from ten to fifteen steps. Fig. 2 shows the tower
and its two curtains on the side of the town; the bridges of
communication are supposed to have been removed. The battlemented
portion at the top is covered with a roof, and open on the side of the
town in order to permit the defenders of the tower to see what was
going on therein, and also to allow of their hoisting up stones and
other projectiles by means of a rope and pulley. Fig. 3 shows the same
tower on the side towards the country; we have added a postern, the
sill of which is sufficiently raised above the ground to necessitate
the use of a scaling or step ladder, to obtain ingress. The postern is
defended, as was customary, by a palisade or barrier, each gate or
postern being provided with a work of this kind."
[Illustration: Fig. 1.--Plan of one of the Towers at Carcassonne.]
[Illustration: Fig. 2.--One of the Towers at Carcassonne, inside view.]
Meanwhile, in western Europe, siegecraft had almost disappeared. Its
perfect development was only possible for an army like that of the
Romans. The Huns and Goths knew nothing of it, and the efforts of
Charlemagne and others of the Frankish kings to restore the art were
hampered by the fact that their warriors despised handicrafts and
understood nothing but the use of their weapons. During the dark ages
the towns of the Gauls retained their old Roman and Visigoth defences,
which no one knew properly how to attack, and accordingly the sieges of
that period dragged themselves out through long years, and if ultimately
successful were so as a rule only through blockade and famine. It was
not until the 11th century that siegecraft was revived in the West on
the ancient lines.
[Illustration: FIG. 3.--One of the Towers at Carcassonne, outside view.]
Castles.
By this time a new departure of great importance had been made in the
seigneurial castle (q.v.), which restored for some centuries a definite
superiority to the defence. Built primarily as strongholds for local
magnates or for small bodies of warriors dominating a conquered country,
the conditions which called them into existence offered several marked
advantages. The defences of a town had to follow the growth of the town,
and would naturally have weak points. It was not to be expected that a
town would develop itself in the manner most suitable for defence; nor
indeed that any position large enough for a town could be found that
would be naturally strong all round. But the site of a castle could be
chosen purely for its natural strength, without regard, except as a
secondary consideration, to the protection of anything outside it; and
as its area was small it was often easy to find a natural position
entirely suited for the purpose. In fact it frequently happened that the
existence of such a position was the _raison d'etre_ of the castle. A
small hill with steep sides might well be unapproachable in every
direction by such cumbrous structures as towers and rats, while the
height of the hill, added to the height of the walls, would be too much
for the besiegers' missiles. If the sides of the hill were precipitous
and rocky, mining became impossible, and the site was perfect for
defence. A castle built under such conditions was practically
impregnable; and this was the cause of the independence of the barons in
the 11th and 12th centuries. They could only be reduced by blockade, and
a blockade of long duration was very difficult in the feudal age.
A very instructive example of 12th-century work is the Chateau Gaillard,
built by Richard Coeur-de-Lion in 1196. This great castle, with ditches
and escarpments cut out of the solid rock, and extensive outworks, was
completed in one year. In the article CASTLE will be found the plan of
the main work, which is here supplemented by an elevation of the donjon
(or keep). The waved face of the inner or main wall of the castle,
giving a divergent fire over the front, is an interesting feature in
advance of the time. So also is the masonry protection of the
machicolation at the top of the donjon, a protection which at that time
was usually given by wooden hoardings. After the death of Richard,
Philip Augustus besieged the chateau, and carried it after a blockade of
seven months and a regular attack of one month. In this attack the tower
at A was first mined, after which the whole of that outwork was
abandoned by the defenders. The outer enceinte was next captured by
surprise; and finally the gate of the main wall was breached by the
pioneers. When this happened a sudden rush of the besiegers prevented
the remains of the garrison from gaining the shelter of the donjon, and
they had to lay down their arms.
Chateau Gaillard, designed by perhaps the greatest general of his time,
exemplifies in its brief resistance the weak points of the designs of
the 12th century. It is easy to understand how at each step gained by
the besiegers the very difficulties which had been placed in the way of
their further advance prevented the garrison from reinforcing strongly
the points attacked.
In the 13th century many influences were at work in the development of
castellar fortification. The experience of such sieges as that of
Chateau Gaillard, and still more that gained in the Crusades, the larger
garrisons at the disposal of the great feudal lords, and the importance
of the interests which they had to protect in their towns, led to a
freer style of design. We must also take note of an essential difference
between the forms of attack preferred by the Roman soldiery and by the
medieval chivalry. The former, who were artisans as well as soldiers,
preferred in siege works the certain if laborious methods of breaching
and mining. The latter, who considered all manual labour beneath them
and whose only ideal of warfare was personal combat, affected the tower
and its bridge, giving access to the top of the wall rather than the rat
and battering-ram. They were also fond of surprises, which the bad
discipline of the time favoured.
[Illustration: FIG. 4.--Donjon, Chateau Gaillard.]
We find, therefore, important progress in enlarging the area of defence
and in improving arrangements for flanking. The size and height of all
works were increased. The keep of Coucy Castle, built in 1220, was 200
ft. high. Montargis Castle, also built about this time, had a central
donjon and a large open enclosure, within which the whole garrison could
move freely, to reinforce quickly any threatened point. The effect of
flanking fire was increased by giving more projection to the towers,
whose sides were in some cases made at right angles to the curtain
walls.
We find also a tendency, the influence of which lasted long after
medieval times, towards complexity and multiplication of defences, to
guard against surprise and localize successful assaults. Great attention
was paid to the "step by step" defence. Flanking towers were cut off
from their walls and arranged for separate resistance. Complicated
entrances with traps and many doors were arranged. Almost all defence
was from the tops of the walls and towers, the loopholes on the lower
storeys being mainly for light and air and reconnoitring. Machicouli
galleries (for vertical defence) were protected either by stone walls
built out on corbels, or by strong timber hoardings built in war time,
for which the walls were prepared beforehand by recesses left in the
masonry. Loopholes and crenelles were protected by shutters. Great care
and much ingenuity were expended on details of all kinds.
Already in the 12th century the engineers of the defence had made
provision for countermining, by building chambers and galleries at the
base of the towers and walls. Further protection for the towers against
the pioneer attack was given by carrying out the masonry in front of the
tower in a kind of projecting horn. This was found later to have the
further advantages of doing away with the dead ground in front of the
tower unseen from the curtain, and of increasing the projection and
therefore the flanking power of the tower itself. The arrangement is
seen in several of the towers at Carcassonne, and has in it the germ of
the idea of the bastion.
[Illustration: FIG. 5.--Plan of Carcassonne, 13th century.]
The defences of Carcassonne, remodelled in the latter half of the 13th
century on the old Visigoth foundations, exemplify some of the best
work of the period. Figs. 5 and 6 (reproduced from Viollet-le-Duc)
show the plan of the defences of the town and castle, and a bird's-eye
view of the castle with its two barbicans. The thick black line shows
the main wall; beyond this are the lists and then the moat. It will be
noted that the wall of the lists as well as the main wall is defended
by towers. There are only two gates. That on the east is defended by
two great towers and a semicircular barbican. The gate of the castle,
on the west, has a most complicated approach defended by a labyrinth
of gates and flanking walls, which cannot be shown on this small
scale, and beyond these is a huge circular barbican in several
storeys, capable of holding 1500 men. On the side of the town the
castle is protected by a wide moat, and the entrance is masked by
another large semicircular barbican. An interesting feature of the
general arrangement is the importance which the lists have assumed.
The slight wooden barricade of older times has developed into a wall
with towers; and the effect is that the besieger, if he gains a
footing in the lists, has a very narrow space in which to work the
engines of attack. The castle, after the Roman fashion, adjoins the
outer wall of the town, so that there may be a possibility of
communicating with a relieving force from outside after the town has
fallen. There were also several posterns, small openings made in the
wall at some height above the ground, for use with rope ladders.
The siegecraft of the period was still that of the ancients. Mining was
the most effective form of attack, and the approach to the walls was
covered by engines throwing great stones against the hoardings of the
parapets, and by cross-bowmen who were sheltered behind light mantlets
moved on wheels. Barrels of burning pitch and other incendiary
projectiles were thrown as before; and at one siege we read of the
carcasses of dead horses and barrels of sewage being thrown into the
town to breed pestilence, which had the effect of forcing a
capitulation.
With all this the attack was inferior to the defence. As Professor
C.W.C. Oman has pointed out, the mechanical application of the three
powers of tension, torsion and counterpoise (in the missile engines) had
its limits. If these engines were enlarged they grew too costly and
unwieldy. If they were multiplied it was impossible on account of their
short range and great bulk to concentrate the fire of enough of them on
a single portion of the wall.
Introduction of gunpowder.
It is difficult to give anything like an accurate account, in a small
space, of the changes in fortification which took place in the first two
centuries after the introduction of gunpowder. The number of existing
fortifications that had to be modified was infinite, so also was the
number of attempted solutions of the new problems. Engineers had not yet
begun to publish descriptions of their "systems"; also the new names and
terms which came into use with the new works were spread over Europe by
engineers of different countries, and adopted into new languages without
much accuracy.
Artillery was in use for some time before it began to have any effect on
the design of fortification. The earliest cannon threw so very light a
projectile that they had no effect on masonry and were more useful for
the defence than the attack. Later, larger pieces were made, which acted
practically as mortars, throwing stone balls with high elevation, and
barrels of burning composition. In the middle of the 15th century the
art of cannon-founding was much developed by the brothers Bureau in
France. They introduced iron cannon balls and greatly strengthened the
guns. In 1428 the English besieging Orleans were entirely defeated by
the superior artillery of the besieged. By 1450 Charles VII. was
furnished with so powerful a siege train that he captured the whole of
the castles in Normandy from the English in one year.
[Illustration: FIG. 6.--Carcassonne Castle and Barbican.]
But the great change came after the invasion of Italy by Charles VIII.
with a greatly improved siege train in 1494. The astonishing rapidity
with which castles and fortified towns fell before him proved the
uselessness of the old defences. It became necessary to create a new
system of defences, and, says Cosseron de Villenoisy, "thanks to the
mental activity of the Renaissance and the warlike conditions prevailing
everywhere, the time could not have been more favourable." There is no
doubt that the engineers of Italy as a body were responsible for the
first advance in fortification. There, where vital and mental energy
were at boiling-point, and where the first striking demonstration of the
new force had been given, the greatest intellects, men such as Leonardo
da Vinci, Michelangelo and Machiavelli, busied themselves over the
problem of defence.
It has been claimed that Albert Durer was the first writer on modern
fortification. This was not so; Durer's work was published in 1527, and
more than one Italian engineer, certainly Martini of Siena and San
Gallo, had preceded him. Also Machiavelli, writing between 1512 and
1527, had offered some most valuable criticisms and general principles.
Durer, moreover, had little influence on the progress of fortification;
though we may see in his ideas, if we choose, the germ of the
"polygonal" system, developed long afterwards by Montalembert. Durer's
work was to some extent a connecting link between the old fortification
and the new. He proposed greatly to enlarge the old towers; and he
provided both them and the curtains with vaulted chambers for guns
(casemates) in several tiers, so as to command both the ditch and the
ground beyond it. His projects were too massive and costly for
execution, but his name is associated with the first practical gun
casemates.
Before beginning to trace the effect of gunpowder on the design of
fortification, it may be noted that two causes weakened the influence of
the castles. First, their owners were slow to adopt the new ideas and
abandon their high strong walls for low extended parapets, and,
secondly, they had not the men necessary for long lines of defence. At
the same time the corporations of the towns had learnt to take an active
part in warfare, and provided trained and disciplined soldiers in large
numbers.
When artillery became strong enough to destroy masonry from a distance
two results followed: it was necessary to modify the masonry defences so
as to make them less vulnerable, and to improve the means of employing
the guns of the defence. For both these purposes the older castles with
their restricted area were little suited, and we must now trace the
development of the fortified towns.
The bulwark.
Probably the first form of construction directly due to the appearance
of the new weapons was the bulwark (_boulevard, baluardo_ or
_bollwerk_). This was an outwork usually semicircular in plan, built
of earth consolidated with timber and revetted with hurdles. Such
works were placed as a shield in front of the gates, which could be
destroyed even by the early light cannon-balls; and they offered at
the same time advanced positions for the guns of the defence. They
were found so useful for gun positions for flanking fire that later
they were placed in front of towers or at intervals along the walls
for that purpose.
This, however, was only a temporary expedient, and we have now to
consider the radical modifications in designs. These affected both the
construction and trace of the walls.
The wall.
The first lesson taught by improved artillery was that the walls
should not be set up on high as targets, but in some manner screened.
One method of doing this in the case of old works was by placing
bulwarks in front of them. In other cases the lists or outer walls,
being surrounded by moats, were already partially screened and
suitable for conversion into the main defence; and as with improved
flanking defence great height was no longer essential, the tops of the
walls were in some cases cut down. In new works it was natural to sink
the wall in a ditch, the earth from which was useful for making
ramparts.
As regards resistance to the effect of shot, it was found that thin
masonry walls with rubble filling behind them were very easily
destroyed. A bank of earth behind the wall lessened the vibration of
the shot, but once a breach was made the earth came down, making a
slope easy of ascent. To obviate this, horizontal layers of brushwood,
timber and sometimes masonry were built into the earth bank, and
answered very well (fig. 7).
[Illustration: FIG. 7.]
[Illustration: FIG. 8.]
[Illustration: FIG. 9.]
Another expedient of still greater value was the use of counterforts.
The earliest counterforts were simply buttresses built _inward_ from
the wall into the rampart instead of _outward_ (fig. 8). Their effect
was to strengthen the wall and make the breaches more difficult of
ascent. An alternative arrangement for strengthening the wall was an
arched gallery built behind it under the rampart (fig. 9). This
construction was in harmony with the idea, already familiar, of a
passage in the wall from which countermines could be started; but it
has the obvious weakness that the destruction of the face wall takes
away one of the supports of the arch. The best arrangement, which is
ascribed to Albert Durer, was the "counter-arched revetment." This
consisted of a series of arches built between the counterforts, with
their axes at right angles to the face of the wall. Their advantage
was that, while supporting the wall and taking all the weight of the
rampart, they formed an obstacle after the destruction of the wall
more difficult to surmount than the wall itself and very hard to
destroy. The counter-arches might be in one, two or three tiers,
according to the height of the wall (figs. 10 and 11, the latter
without the earth of the rampart and showing also a countermine
gallery).
The rampart.
A more important question, however, than the improvement of the
passive defence or obstacle was the development of the active defence
by artillery. For this purpose it was necessary to find room for the
working of the guns. At the outset it was of course a question of
modifying the existing defences at as little cost as possible. With
this object the roofs of towers were removed and platforms for guns
substituted, but this only gave room for one or two guns. Also the
loopholes in the lower storeys of towers were converted into
embrasures to give a grazing fire over the ditch; this became the
commonest method of strengthening old works for cannon, but was of
little use as the resulting field of fire was so small. In some cases
the towers were made larger, with a semicircular front and side walls
at right angles to the curtain. Such towers built at Langres early in
the 16th century had walls 20 ft. thick to resist battering.
[Illustration: FIG. 10.]
Even in new works some attempts were made to combine artillery defence
with pure masonry protection. The works of Albert Durer in theory, and
the bridge-head of Schaffhausen in practice, are the best examples of
this. The Italian engineers also showed much ingenuity in arranging
for the defence of ditches with masonry caponiers. These were
developed from external buttresses, and equally with the casemated
flanking towers of Durer contained the germs of the idea of
"polygonal" defence.
The natural solution, however, which was soon generally adopted, was
the rampart; that is, a bank of earth thrown up behind the wall,
which, while strengthening the wall as already indicated, offered
plenty of space for the disposal of the guns.
The ditch.
The _ditch_, which had only been occasionally used in ancient and
medieval fortification, now became essential and characteristic.
Serving as it did for the double purpose of supplying earth for a
rampart and allowing the wall to be sunk for concealment, it was found
also to have a definite use as an obstacle. Hitherto the wall had
sufficed for this purpose, the ditch being useful mainly to prevent
the besieger from bringing up his engines of attack.
When the wall (or escarp) was lowered, the obstacle offered by the
ditch was increased by revetting the far side of it with a
_counterscarp_. Beyond the counterscarp wall some of the earth
excavated from the ditch was piled up to increase the protection given
to the escarp wall. This earth was sloped down gently on the outer
side to meet the natural surface of the ground in such a manner as to
be swept by the fire from the ramparts and was called the _glacis_.
[Illustration: FIG. 11.]
Now, however, a new difficulty arose. In all times a chief element in
a successful defence has consisted in action by the besieged outside
the walls. The old ditches, when they existed, had merely a slope on
the far side leading up to the ground-level; and the ditch was a
convenient place in which troops preparing for a sortie could assemble
without being seen by the enemy, and ascend the slope to make their
attack. The introduction of the counterscarp wall prevented sorties
from the ditch. At first it was customary, after the introduction of
the counterscarp, to leave a narrow space on the top of it, behind the
glacis, for a patrol path. Eventually the difficulty was met by
widening this patrol path into a space of about 30 ft., in which there
was room for troops to assemble. This was known as the _covered way_.
With this last addition the ordinary elements of a profile of modern
fortification were complete and are exemplified in fig. 12.
[Illustration: FIG. 12.]
The trace.
Up to the gunpowder period the _trace_ of fortifications, that is, the
plan on which they were arranged on the ground, was very simple. It was
merely a question of an enclosure wall adapted to the site and provided
with towers at suitable intervals. The foot of the wall could be seen
and defended everywhere, from the tops of the towers and the machicoulis
galleries. The introduction of ramparts and artillery made this more
difficult in two ways. The rampart, interposed between the defenders and
the face of the wall, put a stop to vertical defence. Also with the
inferior gun-carriages of the time very little depression could be given
to the guns, and thus the top of the enceinte wall, with or without a
rampart, was not a suitable position for guns intended to flank the
ditch in their immediate neighbourhood. The problem of the "trace"
therefore at the beginning of the 16th century was to rearrange the line
of defence so as to give due opportunity to the artillery of the
besieged, both to oppose the besiegers' breaching batteries and later to
defend the breaches. At the outset the latter role was the more
important.
In considering the early efforts of engineers to solve this problem we
must remember that for economical reasons they had to make the best use
they could of the existing walls. At first for flanking purposes
casemates on the ditch level were used, the old flanking towers being
enlarged for the purpose. Masonry galleries were constructed across the
ditch, containing casemates which could fire to either side, and after
this casemates were used in the counterscarps. Some use was also made of
the fire from detached bulwarks. It was soon realized, however, that the
flanking defence of the body of the place ought not to be dependent on
outworks, and that greater freedom was required for guns than was
consistent with casemate defence. The _bulwark_ (which in its earliest
shape suggests that it was in some sort the offspring of the barbican,
placed to protect an entrance) gave plenty of space for guns, but was
too detached for security. The enlarged tower, as an integral part of
the lines, gave security, and its walls at right angles to the curtain
gave direct flanking fire, but the guns in it were too cramped. The
blending of the two ideas produced the _bastion_, an element of
fortification which dominated the science for three hundred years, and
so impressed itself on the imagination that to this day any strong
advanced position in a defensive line is called by that name by
unscientific writers. The word had been in use for a long time in
connexion with extemporized towers or platforms for flanking purposes,
the earliest forms being _bastille_, _bastide_, _bastillon_, and in its
origin it apparently refers rather to the quality of work in the
construction than to its defensive intention.
[Illustration: FIG. 13.--Bastion at Troyes.]
The earliest bastions were modified bulwarks with straight faces and
flanks, attached to the main wall, for which the old towers often
acted as keeps; and at first the terms bulwark and bastion were more
or less interchangeable. Fig. 13, taken from a contemporary MS. by
Viollet-le-Duc, shows a bastion added to the old wall of Troyes about
1528. On the other hand, in fig. 14 (taken from an English MS. of
1559, which again is based on the Italian work of Zanchi published in
1554), we find _a a_ spoken of as "bulwarks" and _b b_ as
"bastilions." The triangular works between the bastilions are
described as "ramparts," intended to protect the curtains from
breaching fire. (We may also notice in this design the broad ditch,
the counterscarp with narrow covered way, and loopholes indicating
counterscarp galleries.)
Towards the end of the 16th century the term "bulwark" began to be
reserved for banks of earth thrown up a little distance in front of the
main wall to protect it from breaching fire, and it thus reverted to its
original defensive intention. The term "bastion" henceforth denoted an
artillery position connected by flanks to the main wall; and the
question of the arrangement of these flanks was one of the main
preoccupations of engineers. Flanks retired, casemated or open, or
sometimes in several tiers were proposed in infinite variety.
[Illustration: FIG. 14.]
Thus, while in the early part of the 16th century the actual
modification of existing defences was proceeding very slowly on account
of the expense involved, the era of theoretical "systems" had begun,
based on the mutual relations of flank and face. These can be grouped
under three heads as follows:--
1. The _cremaillere_ or indented trace: Faces and flanks succeeding
each other in regular order (fig. 15).
[Illustration: FIG. 15.]
[Illustration: FIG. 16.]
2. The _tenaille_ trace: Flanks back to back between the faces (fig.
16). The development of the flanks in this case gives us the _star_
trace (fig. 17).
[Illustration: FIG. 17.]
[Illustration: FIG. 18.]
3. The _bastioned_ trace: Flanks facing each other and connected by
curtains (fig. 18).
In comparing these three traces it will be observed that unless
casemates are used the flanking in the first two is incomplete. Guns on
the ramparts of the faces cannot defend the flanks, and therefore there
are "dead" angles in the ditch. In the bastioned trace there is no
"dead" ground, provided the flanks are so far apart that a shot from the
rampart of a flank can reach the ditch at the centre of the curtain.
The bastioned trace.
Here was therefore the parting of the ways. For those who objected to
casemate fire, the bastioned trace was the way of salvation. They were
soon in the majority; perhaps because the symmetry and completeness of
the idea captivated the imagination. At all events the bastioned trace,
once fairly developed, held the field in one form or another practically
without a rival until near the end of the 18th century. The Italian
engineers, who were supreme throughout most of the 16th century, started
it; the French, who took the lead in the following century, developed
it, and officially never deserted it until late in the 19th century,
when the increasing power of artillery made enceintes of secondary
importance.
It will be useful at this point to go forward a little, with a couple
of explanatory figures, in order to get a grasp of the component parts
of the bastioned trace as ultimately developed, and of its outworks.
[Illustration: FIG. 19.]
In fig. 19 ABCD represents part of an imaginary line drawn round the
place to be fortified, forming a polygon, regular or irregular.
ABC is an _exterior angle_ or angle of the polygon.
BC is an _exterior side_.
_zz_ is an _interior side_.
_abcdefghijk_ is the trace of the _enceinte_.
_bcdef_ is a _bastion_.
_zdef_ is a _demi-bastion_.
_de_ is a _face_ of the bastion.
_ef_ is a _flank_ of the bastion.
_fg_ is the _curtain_.
_bf_ is the _gorge_.
(Two demi-bastions with the connecting curtain make the bastioned
front, _defghi_.)
_zd_ bisecting the _exterior angle_ ABC is the _capital_ of the
bastion.
_xy_ is the _perpendicular_, the proportionate length of which to the
exterior side BC (usually about one-sixth) is an important element of
the trace.
_ef_C is the angle of _defence_.
BC_f_ is the _diminished angle_.
_cde_ is the _flanked angle_ or _salient angle_ of the bastion.
_e_ is the _shoulder_ of the bastion.
_def_ is the _angle of the shoulder_.
_efg_ is the _angle of the flank_.
The line of the escarp is called the _magistral line_ since it
regulates the trace. When plans of fortifications are given without
much detail, this line, with that of the counterscarp and the crest of
the parapet, are often the only ones shown,--the crest of the parapet,
as being the most important line, whence the fire proceeds, being
usually emphasized by a thick black line.
[Illustration: FIG. 20.]
Fig. 20, reproduced from a French engraving of 1705, shows an
imaginary place fortified as a hexagon with bastions and all the
different kinds of outworks then in use. The following is the
explanation of its figuring and lettering.
1. _Flat bastion:_ Placed in the middle of a curtain when the lines of
defence were too long for musketry range.