, ▁although ▁the ▁sh or eline ▁has ▁since ▁re ced ed ▁as ▁a ▁result ▁of ▁land ▁re clam ation ; ▁the ▁river ▁is ▁now ▁about ▁ ▁north ▁of ▁the ▁castle , ▁separated ▁from ▁it ▁by ▁mar sh es . ▁It ▁was ▁constructed ▁by ▁John ▁de ▁Cob ham , ▁ 3 rd ▁Baron ▁Cob ham ▁( d . ▁ 1 4 0 8 ), ▁of ▁nearby ▁Cob ham ▁Hall , ▁whose ▁family ▁had ▁acquired ▁the ▁man or ▁of ▁C ool ing ▁in ▁the ▁mid - 1 3 th ▁century . ▁In ▁ 1 3 7 9 , ▁during ▁the ▁second ▁phase ▁of ▁the ▁H undred ▁Years ' ▁War , ▁a ▁French ▁ra id ▁dev ast ated ▁towns ▁and ▁villages ▁along ▁the ▁Th ames ▁Est u ary . ▁Cob ham ▁appe aled ▁to ▁the ▁Crown ▁for ▁lic ence ▁to ▁fort ify ▁his ▁man or ▁and ▁received ▁permission ▁in ▁February ▁ 1 3 8 0 . ▁The ▁building ▁work ▁was ▁completed ▁by ▁ 1 3 8 5 . ▁ ▁Sur v iving ▁records ▁indicate ▁that ▁the ▁prominent ▁gate house ▁was ▁built ▁by ▁local ▁labour ▁under ▁several ▁master ▁mas ons , ▁including ▁Thomas ▁C rom pe , ▁William ▁Sh arn all ▁and ▁Thomas ▁W rek , ▁with ▁the ▁king ' s ▁master ▁m ason ▁Henry ▁Ye ve le ▁taking ▁a ▁super vis ory ▁role . ▁The ▁castle ▁is ▁of ▁particular ▁importance ▁as ▁the ▁earliest ▁English ▁castle ▁designed ▁for ▁the ▁use ▁of ▁gun pow der ▁weapons . ▁Lord ▁Cob ham ' s ▁instructions ▁to ▁his ▁mas ons ▁include