1 3 5 ▁by ▁Alexander , ▁Bishop ▁of ▁Lincoln , ▁in ▁a ▁mot te ▁and ▁ba iley ▁design . ▁The ▁castle ▁was ▁later ▁conf isc ated ▁from ▁Alexander ▁by ▁King ▁Stephen ▁in ▁ 1 1 3 9 , ▁but ▁was ▁returned ▁to ▁the ▁bishop ▁later ▁that ▁year ▁and ▁mostly ▁remained ▁in ▁the ▁hands ▁of ▁later ▁b ish ops ▁until ▁ 1 5 4 7 . ▁The ▁castle ▁was ▁protected ▁by ▁a ▁castle - guard ▁drawn ▁from ▁est ates ▁around ▁Ban bury . ▁The ▁castle ▁was ▁strength ened ▁between ▁ 1 2 0 1 – 7 ▁during ▁the ▁reign ▁of ▁King ▁John . ▁ ▁The ▁castle ▁was ▁subsequently ▁completely ▁re built ; ▁earlier ▁histor ians ▁had ▁concluded ▁that ▁the ▁re building ▁occurred ▁in ▁the ▁late ▁ 1 3 th ▁century , ▁but ▁arch ae ological ▁exc av ations ▁in ▁the ▁ 1 9 7 0 s ▁demonstrated ▁that ▁the ▁work ▁occurred ▁between ▁ 1 2 2 5 – 5 0 . ▁The ▁new ▁castle ▁had ▁a ▁concent ric ▁pent ag onal ▁plan , ▁with ▁thick , ▁emb ank ed ▁walls . ▁The ▁castle ▁had ▁a ▁gate house , ▁although ▁later ▁damage ▁means ▁that ▁its ▁design ▁remains ▁uncertain . ▁Histor ian ▁John ▁Ken y on ▁con cludes ▁that ▁Ban bury ▁Castle ▁is ▁" remark able ▁for ▁its ▁early ▁concent ric ▁shape ", ▁which ▁is ▁usually ▁seen ▁in ▁somewhat ▁later ▁cast les ▁such ▁as ▁Har le ch ▁or ▁Be aum aris . ▁By ▁the ▁second ▁half ▁of ▁the ▁ 1 3 th ▁century ▁the ▁castle ▁was