 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 . 
