e shell-keep and rectangular tower) soon
penetrated to Europe, and Alnwick Castle (1140-1150) shows the influence
of the new system. But the finest of all castles of the middle ages was
Richard Coeur de Lion's fortress of Chateau Gaillard (1197) on the Seine
near Les Andelys. Here the innermost ward was protected by an elaborate
system of strong appended defences, which included a strong
_tete-de-pont_ covering the Seine bridge (see Clark, i. 384, and Oman,
p. 533). The castle stood upon high ground and consisted of three
distinct enceintes or wards besides the keep, which was in this case
merely a strong tower forming part of the innermost ward. The donjon was
rarely defended _a outrance_, and it gradually sank in importance as
the outer "wards" grew stronger. Round instead of rectangular towers
were now becoming usual, the finest examples of their employment as
keeps being at Conisborough in England and at Coucy in France. Against
the relatively feeble siege artillery of the 13th century a well-built
fortress was almost proof, but the mines and the battering ram of the
attack were more formidable, and it was realized that corners in the
stonework of the fortress were more vulnerable than a uniform curved
surface. Chateau Gaillard fell to Philip Augustus in 1204 after a
strenuous defence, and the success of the assailants was largely due to
the wise and skilful employment of mines. An angle of the noble keep of
Rochester was undermined and brought down by John in 1215.

[Illustration: FIG. 5.--Krak-des-Chevaliers: View.]

[Illustration: FIG. 6.--Chateau Gaillard.

A. _High Angle Tower_
B.B. _Smaller Side Towers_
C.C. D.D. _Corner Towers_
E. _Outer Enceinte, or Lower Court_
F. _The Well_
G.H. _Buildings in the Lower Court_
I. _The Moat_
K. _Entrance Gate_
L. _The Counterscarp_
M. _The Keep_
N. _The Escarpment_
O. _Postern Tower_
P. _Postern Gate_
R.R. _Parapet Walls_
S. _Gate from the Escarpment_
T.T. _Flanking Towers_
V. _Outer Tower_
X. _Connecting Wall_
Y. _The Stockade in the River_
Z.Z. _The Great Ditches_]

The next development was the extension of the principle of successive
lines of defence to form what is called the "concentric" castle, in
which each ward was placed wholly within another which enveloped it;
places thus built on a flat side (e.g. Caerphilly Castle) became for the
first time more formidable than strongholds perched upon rocks and
hills such