 The only substantial exception is the medieval Milton Chantry, the oldest building in Gravesend, which was used as a barracks.The lower Thames was of great strategic significance as the location of major military installations including the dockyard and arsenal at Woolwich, the powder magazine at Purfleet, and the victualling stores and shipyard at Deptford.In addition, it was essential to ensure that an enemy could not make a landing along the Thames, which offered a potential land corridor to London.