St Mawes Castle is an artillery fort constructed by Henry VIII near Falmouth, Cornwall, between 1540 and 1542. It formed part of the King's Device programme to protect against invasion from France and the Holy Roman Empire, and defended the Carrick Roads waterway at the mouth of the River Fal. The castle was built under the direction of Thomas Treffry to a clover leaf design, with a four-storey central tower and three protruding, round bastions that formed gun platforms. It was initially armed with 19 artillery pieces, intended for use against enemy shipping, operating in partnership with its sister castle of Pendennis on the other side of the estuary. During the English Civil War, St Mawes was held by Royalist supporters of King Charles I, but surrendered to a Parliamentary army in 1646 in the final phase of the conflict.
The castle continued in use as a fort through the 18th and 19th centuries. In the early 1850s, fears of a fresh conflict with France, combined with changes in military technology, led to the redevelopment of the fortification.