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.

Brought back into service in the Second World War, naval artillery and an anti-aircraft gun were installed at the castle to defend against the risk of German attack.

The castle has elaborate, carved 16th-century decorations including sea monsters and gargoyles, and the historian Paul Pattison has described the site as "arguably the most perfect survivor of all Henry's forts".

[2] St Mawes Castle was built as a consequence of 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.

[3] Basic 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.

[8] In response, Henry issued an order, called a "device", in 1539, giving instructions for the "defence of the realm in time of invasion" and the construction of forts along the English coastline.

[9] The stretch of water known as Carrick Roads at the mouth of the River Fal was an important anchorage serving shipping arriving from the Atlantic and the Mediterranean, and plans were made to protect it with five castles.

[11] The two castles' guns could provide overlapping fire across the water, while St Mawes also overlooked a separate anchorage on the eastern side of the estuary.

[14][a] The clover leaf shaped castle, with an additional small blockhouse at the water's edge below, was armed with 19 artillery pieces - a demi-cannon, a demi-culverin, a demi-sling, five slings, four portpieces and seven bases - along with 12 large hagbusshes, a form of arquebus.

[31] The growing town of Falmouth was a strategically important part of their supply routes to the Continent, while Carrick Roads formed a base for Royalist piracy in the English Channel.

[32] This decision has been put down to a result of war-weariness, the large numbers of Parliamentary troops facing them and the generous surrender terms on offer, although the 19th-century historian Samuel Oliver also suspected that Bonithon might have had Parliamentarian sympathies.

[36] Richard's son, Sir Vyel Vyvyan, became captain in turn on his father's death, but he had no heirs and separated the castle's lands from the captaincy, selling them to John Granville, the Earl of Bath.

A review by Colonel Christian Lilly in 1714 reported that the fortification was in a satisfactory condition, and in the 1730s, St Mawes was equipped with 17 artillery pieces, including six 24-pounder (11 kg) cannons, mostly positioned in the batteries beneath the Henrician castle.

[38] Britain's wars with France in the late-18th century made the defence of Falmouth critical and from 1775 until 1780 the local militia was called up to defend St Mawes.

[41] The poet Lord Byron, visiting in 1809, complained that St Mawes was "extremely well calculated for annoying every body except an enemy", and commented that the fort was garrisoned by only one, elderly man.

[44] The Tudor office of the captaincy was abolished in 1849, with the death of the final incumbent, Sir George Nugent, and the command of the garrison became a regular military appointment.

[47] Fresh concerns about France rose in the 1880s, and an electrically operated minefield was laid across Carrick Roads in 1885, jointly controlled from St Mawes and Pendennis.

[48] Additional contact mines were added, forcing incoming vessels to sail into a channel alongside St Mawes, illuminated with electric search lights.

[59] The design allowed for multiple levels of artillery, and may have been influenced by the contemporary work of the Moravian engineer, Stefan von Haschenperg, on some of the other Device Forts constructed during this period.

[68] The bridge across the moat leads into the second storey, which originally had four chambers with fireplaces and windows, linked by a central corridor; this area may have been used by the castle's officers, and to house an enlarged garrison in an emergency.

[80] The Grand Sea Battery was served by a 19th-century magazine for holding gunpowder, approximately 35 by 18 feet (10.7 by 5.5 m) with stone walls and bomb-proof brick roof, topped with turf to help to protect against incoming shells.