ge . ▁B ole y ▁Hill ▁is ▁a ▁natural ▁out c rop ▁of ▁rock , ▁and ▁could ▁have ▁acted ▁as ▁a ▁mot te . ▁ ▁The ▁castle ▁built ▁by ▁Bishop ▁G und ulf ▁in ▁the ▁late ▁ 1 2 th   century ▁was ▁en closed ▁by ▁a ▁stone ▁wall . ▁Situ ated ▁in ▁the ▁south - west ▁corner ▁of ▁the ▁city , ▁the ▁castle ▁used ▁the ▁remains ▁of ▁the ▁Roman ▁town ▁walls ▁as ▁found ations . ▁The ▁circuit ▁had ▁at ▁least ▁one ▁tower ; ▁it ▁was ▁replaced ▁in ▁the ▁ 1 4 th   century . ▁The ▁original ▁gateway ▁was ▁rad ically ▁alter ed ▁in ▁the ▁ 1 3 th ▁or ▁ 1 4 th   century . ▁From ▁across ▁the ▁River ▁Med way , ▁the ▁tw in ▁land marks ▁of ▁Ro chester ' s ▁castle ▁and ▁c athedral ▁would ▁have ▁domin ated ▁the ▁medieval ▁landscape , ▁symbol ic ▁of ▁the ▁authority ▁of ▁the ▁church ▁and ▁nob ility ▁in ▁the ▁period . ▁Most ▁cast les ▁were ▁built ▁by ▁sec ular ▁nobles , ▁but ▁the ▁work ▁by ▁G und ulf ▁and ▁his ▁successor ▁Cor be il ▁provide ▁examples ▁of ▁the ▁role ▁of ▁the ▁church ▁in ▁castle ▁building . ▁ ▁According ▁to ▁military ▁historian ▁Allen ▁Brown ▁Ro chester ' s ▁keep ▁is ▁" am ong ▁the ▁fin est ▁and ▁oldest ▁in ▁all ▁England ". ▁Since ▁its ▁construction ▁it ▁has ▁under g one ▁limited ▁alter ation , ▁aside ▁from ▁the ▁re building ▁of ▁one ▁corner , ▁and ▁although ▁now ▁in ▁a ▁state ▁of ▁ru in ▁it