The castles of the eleventh and twelfth centuries, whether in Normandy or in England, were of two distinct types,—those with the rectangular and those with the shell keep. The former type was almost always employed when the site selected was a new one, the latter where the site was old, and where there existed a “motte” or mound. There are exceptions to this; that is to say, the rectangular keep is occasionally found on an old site, but the shell keep is never found on a new one. The distinction was mainly due to the fact that the massive heavy tower could only be safely founded upon solid ground, whereas the lighter and more widely-distributed weight of the shell keep was better suited to that which was artificial. The shell keep was the most numerous of the two; but the tower type, being of a more solid and more durable character, has lasted longest, and is at this time so much the most common that it has been designated by writers of authority as the type, instead of as but one of the two types of a Norman keep. The rectangular and the shell keep never occur in the same castle; and, as a rule, where there is a mound there is no rectangular keep. The only known exceptions to this rule are at Christchurch, Guildford, Clun, Saffron Walden, Mileham, Bungay, and Bramber. At Christchurch and Mileham the mound is low, and the keep walls seem to be carried through it to the solid ground. At Guildford, as at Clun, where the mound is large, the keep is built on its slope, so that the lower end and one-third of the contiguous sides are seen to rest on the solid ground, and thus effectually counteract the tendency of the upper part to slide down. Walden has not been carefully examined, nor is it known whether the foundations at Bungay are carried down to the solid earth. At Bramber there is an oblong natural hill, surrounded by a very deep ditch, in part artificial, while at one end of the hill is a square keep, and near the other a small mound. At Llwchwr, in South Wales, a small square tower stands upon a low and small mound, mixed up with the bank of a Roman camp. The tower, indeed, is not actually Norman, though no doubt early, and it most probably descends to the natural ground. The large rectangular tower at Oxford is not the keep, but a mural tower, though of unusual bulk. The keep there was a shell, and crowned the mound. At Kenilworth there seems to have been a small mound, which has been included within the keep, the walls of which rest upon the natural rock. The mound in this way fills up the basement of the keep, the ground-floor of which is thus raised 12 feet to 15 feet above the exterior level, as in the shell keep of Berkeley and in some degree at Pontefract.