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.

St Catherine's Castle was built as a consequence of the international tensions between England, France and the Holy Roman Empire in the final years of the reign of King Henry VIII.

Traditionally the Crown had left coastal defences to the local lords and communities, only taking a modest role in building and maintaining fortifications, and while France and the Empire remained in conflict with one another, maritime raids were common but an actual invasion of England seemed unlikely.

[1] Modest defences, based around simple blockhouses and towers, existed in the south-west and along the Sussex coast, with a few more impressive works in the north of England, but in general the fortifications were very limited in scale.

[2] In 1533, Henry broke with Pope Paul III in order to annul his long-standing marriage to Catherine of Aragon and remarry.

[6] In response to this situation, a small, D-shaped stone fortification was built to protect Fowey Harbour in Cornwall, then an important centre for trade.

[11] During the English Civil War of the 1640s, it was held by the Royalist supporters of King Charles I against Parliament; in 1684 the local burgesses reported that the fortification was in a "runious" state.

[15] In 1887, the castle was equipped with 64-pound (29 kg) rifled, muzzle-loading artillery pieces, supported by volunteer forces and used for training purposes, but the weaponry became obsolete and the site had fallen out of use again by the end of the century.

[15] It is surrounded by cliffs and a curtain wall, enclosing a semi-circular area around 500 square metres (5,400 sq ft) in size, incorporating slits for firing muskets.