 St Catherine 's Castle ( Cornish : Kastel S. <unk> ) is a Henrician castle in Cornwall , built by Thomas Treffry between approximately 1538 and 1540 , in response to fears of an invasion of England by France and the Holy Roman Empire . The D @-@ shaped , stone fortification , equipped with five gun @-@ ports for cannon , overlooked the mouth of the River Fowey in Cornwall . It was protected by a curtain wall and the surrounding cliffs . The castle remained in use for many years until it was closed at the end of the Napoleonic Wars in 1815 . Brought back into service in 1855 during the Crimean War , it was fitted with two new artillery positions , but it soon became obsolete and was abandoned . During the Second World War the castle was refortified and used to house a battery of naval guns , protecting the coast against the threat of German attack . At the end of the conflict the castle was restored to its previous condition and is now managed by English Heritage as a tourist attraction . 
