 When the castle 's outer walls were eventually breached , the defenders retreated to the relative safety of the keep . It too withstood the efforts of the siege engines , and once again John turned to mining to bring down the walls . The mine was dug beneath the south @-@ east corner of the keep . A letter sent from Rochester on 25 November offers insight into the methods of medieval siegecraft . John ordered Hugh de Burgh to " send to us with all speed by day and night forty of the fattest pigs of the sort least good for eating to bring fire beneath the tower " . The wooden props supporting the tunnel dug beneath the keep were set alight to collapse the mine , bringing down one corner of the keep . Still the garrison held out and sought safety behind the stone partition or cross @-@ wall in the keep , abandoning half the building . The Barnwell chronicler remarked that " for such was the structure of the stronghold that a very strong wall separated the half that had fallen from the other " . 
