VALLUM, a term applied either to the whole or a portion of the
fortifications of a Roman camp. It is derived from _vallus_ (a
stake), and properly means the palisade which ran along the outer
edge of the agger, but it very frequently includes the agger also.
The _vallum_, in the latter sense, together with the _fossa_ or ditch
which surrounded the camp outside of the _vallum_, formed a complete
fortification. The _valli_ (χάρακες), of which the _vallum_, in
the former and more limited sense, was composed, are described by
Polybius and Livy, who make a comparison between the _vallum_ of the
Greeks and that of the Romans, very much to the advantage of the
latter. Both used for _valli_ young trees or arms of larger trees,
with the side branches on them; but the _valli_ of the Greeks were
much larger and had more branches than those of the Romans, which
had either two or three, or at the most four branches, and these
generally on the same side. The Greeks placed their valli in the
agger at considerable intervals, the spaces between them being filled
up by the branches; the Romans fixed theirs close together, and made
the branches interlace, and sharpened their points carefully. Hence
the Greek vallus could easily be taken hold of by its large branches
and pulled from its place, and when it was removed a large opening
was left in the vallum. The Roman vallus, on the contrary, presented
no convenient handle, required very great force to pull it down,
and even if removed left a very small opening. The Greek valli were
cut on the spot; the Romans prepared theirs beforehand, and each
soldier carried three or four of them when on a march. They were
made of any strong wood, but oak was preferred. The word _vallus_
is sometimes used as equivalent to _vallum_. In the operations of
a siege, when the place could not be taken by storm, and it became
necessary to establish a blockade, this was done by drawing defences
similar to those of a camp round the town, which was then said to be
_circumvallatum_. Such a circumvallation, besides cutting off all
communication between the town and the surrounding country, formed
a defence against the sallies of the besieged. There was often a
double line of fortifications, the inner against the town, and the
outer against a force that might attempt to raise the siege. In this
case the army was encamped between the two lines of works. This
kind of circumvallation, which the Greeks called ἀποτειχισμός and
περιτειχισμός, was employed by the Peloponnesians in the siege of
Plataeae. Their lines consisted of two walls (apparently of turf)
at the distance of 16 feet, which surrounded the city in the form
of a circle. Between the walls were the huts of the besiegers. The
wall had battlements (ἐπάλξεις), and at every tenth battlement
was a tower, filling up by its depth the whole space between the
walls. There was a passage for the besiegers through the middle
of each tower. On the outside of each wall was a ditch (τάφρος).
This description would almost exactly answer to the Roman mode of
circumvallation, of which some of the best examples are that of
Carthage by Scipio, that of Numantia by Scipio, and that of Alesia
by Caesar. The towers in such lines were similar to those used in
attacking fortified places, but not so high, and of course not
moveable. [TURRIS.]