This coastal plan is drawn to a scale of 3 inches to the mile, unlike most inland drawings, which are two inches to the mile. The larger scale reflects greater concern for the vulnerability of this area. The Solent gives access to the ports of Portsmouth and Southampton, making it a particularly attractive avenue for naval invasion. Hurst Castle is marked in black and red at the narrow entrance to the Solent. Built by Henry VIII as part of a defensive chain of fortresses, it is sited where the ebb and flow of the tides create particularly strong currents, providing an excellent natural defence against would-be invaders. The castle was modernised during the Napoleonic Wars. To the right of the castle, salt marshes extend towards Lymington. The saltworks, shown by blue squares, once supplied most of the west of England. A signal house is noted on Christchurch Head.