Sandgate was taken by Parliament in 1642 at the start of the first English Civil War, and was seized by Royalist rebels in the second civil war of 1648. The castle was extensively redesigned between 1805 and 1808 during the Napoleonic Wars. The height of the castle was significantly reduced and the keep was turned into a Martello tower; when the work was completed, it was armed with ten 24-pounder (11 kg) guns and could hold a garrison of 40 men.