T. E. Lawrence’s Undergraduate Dissertation – Page 200

Progress in military architecture in Europe depended on the elimination or at least the modification of the square keep form, and accordingly from 1150 there are on record many and varied attempts to improve it, or to give it less prominence in the scheme of defence. The idea that a donjon or last resort of some kind was absolutely necessary persisted strongly through the greater part of the thirteenth century : probably only the introduction of cannon drove it from the field. Long before this time, however, the capture of the outer and inner wards of the castle had meant the surrender of the whole. Modification of the Norman keep took two forms. In the one case a donjon was retained as the more important part of the castle, though its shape was not a perfect square with underfended angles : in another the great keep of passive resistance was swept away entirely, and a light shell keep, with or without a little tower inside it, took its place. In the second instance as time went on the size and importance of the shell increased : until it formed the inner ward of the castle and the tower inside became the donjon. Château Gaillard is the finished result of this process. In other over-grown shell keeps there is no trace of any tower within them at all. In fact the donjon idea has been eliminated. Château Gaillard did not attain to this pitch of perfection, though in practice its keep proved useless.