The keeps were massive rectangular structures several storeys in height, with walls of great thickness. Their size varied according to the requirements of the owner. Some were about 90 ft. square, others but 30 or 40 ft. They were not necessarily exactly square, but, as a rule, their sides were of nearly equal length. The White Tower of the Tower of London, begun by order of the Conqueror in the later years of the eleventh century, measures 118 by 107 ft. The keep of Rochester Castle, built about 1130, is 70 ft. square. Castle Hedingham in Essex, built about the same time, is 60 by 55 ft.; the keep of Dover Castle (about 1154) is 90 ft. square; Castle Rising (Fig. 1), probably a few years later in date, is 75 by 60 ft.; Kenilworth, dating from the third quarter of the century, is 87 by 54 ft.; while the Peak Castle in Derbyshire, erected about 1176, measures some 40 by 36 ft. These are all outside measurements, and as the walls were very thick, seldom less than 8 ft., and sometimes as much as 16 or 20 ft., the available space within them was much less than their total area. Nevertheless, after deducting the thickness of the walls, there remained in the largest such huge rooms as that in the Tower of London, 90 ft. long by 37 ft. wide; in the medium-sized, such as Hedingham, rooms 38 by 31 ft.; while in the smallest, such as the Peak Castle, the space was 22 by 19 ft., equal to the drawing-room of an ordinary house of the present day. But although the rooms were spacious, they were few in number, and badly lighted. As a rule there was but one room on each floor; some of the more important, however, such as Rochester and Castle Rising, had two large rooms on each floor and one or two smaller, but this was the exception rather than the rule. Occasionally a chapel was added; sometimes it occupied part of the floor space inside the walls; sometimes, as at Coningsburgh, it was contrived within the thickness of the wall itself, augmented by hollowing out one of the huge buttresses. But the chapel was always small--space was too valuable for it to be otherwise; and it was used not only for sacred purposes, but also not infrequently as a private room for the lord.