Many of the existing Norman keeps were founded in the reigns of William I. and William II., but were rebuilt at a later period. These keeps were usually square or rectangular towers of stone (not having much height in proportion to the breadth), with small, slightly projecting square turrets at each angle, and one or more flat buttresses up the centre of each face, with a dividing wall passing up through the centre of the building from the ground. (Pl. 22, Fig. 3, B C.) Comparatively speaking, the windows were generally small, and the walls exceedingly thick. Those of the White Tower are 15 ft., and of Carlisle Castle 16 ft. thick. The connecting passages and staircases were constructed in the thickness of the masonry. Norwich Castle, for its size, is a perfect type of the square Norman keep, and Castle Hedingham is another. Many magnificent stone keeps were built or rebuilt in the reign of Henry I., such as Rochester, Newark, Corfe, and Chepstow. Henry II. was also a great builder of keeps, and those of Dover, Canterbury, Scarborough, and Newcastle are shown by the Pipe Rolls to have been his work. The solidarity of the keep made it impregnable against the siege operations of the day. Such a building could not be battered down, and at best it could only be injured by undermining. This was done by removing the earth from an angle of the building and gradually introducing wooden props. A fire was then kindled about them, and as the props burnt through, the wall fell. The square keep was followed by the polygonal and the round ones. Coningsburgh is circular, Berkeley is circular flanked by four towers, and Oxford is polygonal. The base of the keep was generally “battered”--_i.e._, sloped outwards (see Fig. 1)--to give a firmer foundation and also that it might better withstand the operations of the sapper. Very few of the existing keeps have openings in the lower storeys, which were used either as _dungeons_ or _store rooms_, and were only accessible by a trap-door from above. Generally there were two floors, occasionally three floors above the basement. One of these floors is assumed to have been the _hall_, and in the larger keeps the floor above it may have been reserved for the use of the ladies of the household. In the latter half of the twelfth century small mural chambers, probably for use as bedrooms, became more frequent. In many keeps there were _chapels_, but every one contained a _well_, so that when besieged the occupants would not have to depend on outside sources for the supply of water. Dover Castle still has a well which is capable of supplying fresh water, although the keep is on the summit of a high cliff.