But the Normans introduced also two other types of castle. The one was
adopted where they found a natural rock stronghold which only needed
adaptation, as at Clifford, Ludlow, the Peak and Exeter, to produce a
citadel; the other was a type wholly distinct, the high rectangular
tower of masonry, of which the Tower of London is the best-known
example, though that of Colchester was probably constructed in the 11th
century also. But the latter type belongs rather to the more settled
conditions of the 12th century when haste was not a necessity, and in
the first half of which the fine extant keeps of Hedingham and Rochester
were erected. These towers were originally surrounded by palisades,
usually on earthen ramparts, which were replaced later by stone walls.
The whole fortress thus formed was styled a castle, but sometimes more
precisely "tower and castle," the former being the citadel, and the
latter the walled enclosure, which preserved more strictly the meaning
of the Roman _castellum_.
Reliance was placed by the engineers of that time simply and solely on
the inherent strength of the structure, the walls of which defied the
battering-ram, and could only be undermined at the cost of much time and
labour, while the narrow apertures were constructed to exclude arrows or
flaming brands.
[Illustration: FIG. 2.--Vertical section of rectangular Norman Keep
(Tower of London).]
[Illustration: From Oman's _History of the Art of War_, by permission of
Methuen & Co.