The keep wall is from 7 feet to 14 feet thick, and at the base of the foundation sometimes 20 feet. That of Colchester is reputed to be 30 feet. That of the Tower of London is said to have taken six weeks to pierce, with all modern tools and appliances. The lower 14 feet of the keep of Newcastle is thought to be solid. The wall usually diminishes in thickness as it rises, sometimes by external sets-off of 6 inches, more commonly by an internal step of 1 foot at each floor level. Occasionally the exterior face slopes inward or batters, but this is unusual. The summit at the level of the allure or battlement walk is seldom less than 6 feet, and often 7 feet or 8 feet thick. Within, the larger keeps are divided by a cross wall, usually ascending to the summit, and pierced in each floor. Sometimes this wall is confined to the basement and first floor. Kenilworth, a large keep, had no cross wall; Norwich and Canterbury had two, and some have chambers walled off at the ends by secondary cross walls, as Castle Rising, Wolvesey, Colchester, and the White Tower. At Bowes two walls divided the basement into three chambers. Usually these dividing-walls were pierced by doorways, but the openings in the main floor were larger. At Scarborough was, and at Hedingham still is, a single large arch; at Rochester and at Middleham are several arches. At Porchester are only small doorways. Where there is no cross wall its place was, no doubt, supplied by posts of timber. The smaller keeps have a basement and a first floor; the larger, a second and third floor,—the latter being often an early addition.