 In the south Gundulf 's wall survived into the modern period , although has now been dismantled . The current wall is a 19th @-@ century replacement . At the eastern end of this wall , near the southern corner of the castle , is a two @-@ storey rounded tower 30 feet ( 9 @.@ 1 m ) in diameter dating from the early 13th century . It was built to fill the breach in the curtain wall caused when John 's army besieged the castle and to reinforce a weak point in the defences . The section from the tower to the location of the former main gatehouse in the north @-@ east dates from about 1367 to 1370 . Two towers were built along the wall , each two storeys high and again using Kentish Ragstone . The one nearest the keep is relatively plain and the northernmost one more elaborate . The latter was intended for use as a residence and in the modern period was converted into a cottage . The wall between these two towers was reduced in the modern period , possibly to give a better view of the cathedral . Apart from the west side , the castle was surrounded by a ditch , much of which has since been filled in . 
