ot 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.

Fig. 3.--Berkeley Castle, late Norman Shell-Keep.]

[Illustration: From Oman's _History of the Art of War._

FIG. 4.--Krak-des-Chevaliers: Plan.]

At this stage the crusades, and the consequent opportunities afforded to
western engineers of studying the solid fortresses of the Byzantine
empire, revolutionized the art of castle-building, which henceforward
follows recognized principles. Many castles were built in the Holy Land
by the crusaders of the 12th century, and it has been shown (Oman, _Art
of War: the Middle Ages_, p. 529) that the designers realized, first,
that a second line of defences should be built within the main
enceinte, and a third line or keep inside the second line; and secondly,
that a wall must be flanked by projecting towers. From the Byzantine
engineers, through the crusaders, we derive, therefore, the cardinal
principle of the mutual defence of all the parts of a fortress. The
_donjon_ of western Europe was regarded as the fortress, the outer walls
as accessory defences; in the East each envelope was a fortress in
itself, and the keep became merely the last refuge of the garrison, used
only when all else had been captured. Indeed the keep, in several
crusader castles, is no more than a tower, larger than the rest, built
into the enceinte and serving with the rest for its flanking defence,
while the fortress was made strongest on the most exposed front. The
idea of the flanking towers (which were of a type very different from
the slight projections of th