The Keep itself was essentially a new feature in the art of fortification, a medieval method of resisting the special form of attack prevailing at that period. The enclosure was directly derived from the rectangular _castra_ of Roman times, descended through the Anglo-Saxon burh and the Norman bailey. Probably of all the military structures which the world has seen, the Rectangular Keep is the grandest in impressive appearance and dimensions, combined as it is with simplicity of outline; it is also the most durable in workmanship by its adamantine strength and structural proportions. The walls are generally from 8 to 14 feet thick, and, at the base, sometimes even 20 feet, while a few still standing are reputed to have the ground floor solid. The enormous thickness of walls in medieval buildings must not always be taken as an indication of strength; in a large number of cases they consist of two walls at some distance apart, with the intermediate space filled in with rubble and a certain amount of mortar, generally inferior in quality, so that at times when the outer casing is pierced, the interior core pours out through the opening like grain from a sack. They afforded, however, facilities for the construction of passages in the wall itself, and also for small chambers, while the exterior portion of the wall was invariably strengthened by flat pilaster buttresses. The entrances to these Keeps were usually on the first floor, access being gained by means of a ladder or wooden gangway, the doorway being of small dimensions. A series of narrow vertical slits in the walls, splayed out into embrasures inside, served the purpose of windows, and also as oillets or arbalesteria, for the discharge of arrows and bolts. Later examples of the Keep are furnished with forebuildings adapted to protect the vulnerable portion, the entrance. These forebuildings were especially designed to present unusual difficulties of penetration; drawbridges, meurtriers, oubliettes, and other devices being opposed to intruders, while passages leading to every spot except those desired were constructed in the walls to mislead and divert attacks from inrushing assailants. One of the best examples is that at Newcastle-upon-Tyne, built _c._ 1172; it has two towers and contains a chapel, the entrance to the Keep itself being from the roof which forms an open platform. [Illustration: FOREBUILDING OF THE KEEP, BERKELEY.]