The Nore was a sandbar—really the extremity of a vast region of shifting sands deposited round the place where the Thames and the Medway joined together and emptied into the sea. A Buoy had always been anchored there, a few miles off Sheerness Fort, to warn incoming ships, and to force them to choose between going to port—which would, God and tides willing, take them up the Medway, under the guns of Sheerness Fort and then of Castle Upnor, and eventually to Rochester and Chatham—or to starboard, which set them on the way up the Thames to London. Anglesey’s makeshift fleet had been neither the first nor the last invasion force to use that buoy as a rallying-point. The Dutch had done it a few years later. In fact, one of the peculiar duties assigned to the naval ships in this part of the river was to sail up to the Buoy and blow it out of the water whenever serious trouble loomed, so that foreign invaders could not find it.