When King Henry VIII left the Catholic church in 1538 [King Henry abandoned the Catholic church because of the Pope's refusal to grant him a divorce from his first wife, Catherine of Aragon. ] , there was, in Britain, a very serious threat of invasion. Francis I of France and Emperor Charles V of Spain [Emperor Charles V was the nephew of Catherine of Aragon. ] signed a treaty, and the Pope encouraged both nations to invade and attack. Although internal matters in both nations prevented them from invading straight away, the threat remained. So in the 1540s, an extensive campaign to fortify England was begun, with many of these forts on or around the Isle of Wight.

In 1497, the first dry-dock [A dry-dock is a place where the water can be drained in order to fix the hull of a ship. ] in Britain had been built in Portsmouth, and this made it the home of the Royal Navy. It also made it one of the prime targets for invasion, and Henry VIII's strategy was to defend all harbours, and to deny the enemy any harbour or anchorage that could be used to establish a base from which to organise a