 According to military historian Allen Brown Rochester's keep is "among the finest and oldest in all England". Since its construction it underwent limited alteration, aside from the rebuilding of one corner, and although now in a state of ruin it remains significantly intact and is considered one of the most important surviving 12th-century keeps in England and France. The keep was richly decorated with hangings and furnishings. Dating from the second quarter of the 12th century, it is Rochester Castle's dominant feature. It had a square plan, and measures 70 by 70 feet (21 by 21 m) externally with pilaster buttresses at each corner. The keep was built in the castle's southern corner, close to the curtain wall. The primary building material was local Kentish Ragstone, although Caen stone used to face the keep was imported from Normandy. The same material was imported for the Tower of London's White Tower in the 11th century. The tops of the turrets rise 125 feet (38 m) above the ground, 12 ft above the battlements. Below the latter are rectangular holes, marking where wooden hoarding would have been attached. Though 12 ft thick at the base, the walls of the keep taper to 10 ft at the top. It is the tallest keep in England, and only those at Dover, the Tower of London, Colchester, and Norwich are larger. During John's siege of Rochester in 1215, the south-east corner collapsed; during Henry III's reign it was rebuilt as a cylinder. The windows increase in size higher up the walls, although only the uppermost were decorated. A spiral staircase in the north-east corner provided access to all floors, and another in the south-west corner went from the first floor to the top floor. The north-west corner tower contains small chambers, and the south-east probably had a similar layout before it was rebuilt.
