It formed part of the King's Device programme to protect against invasion from France and the Holy Roman Empire and defend Southampton Water as it met the Solent.

[1] Calshot 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 local lords and communities, only taking a modest role in building and maintaining fortifications, and while France and the Empire remained in conflict, maritime raids were common but an actual invasion of England seemed unlikely.

[2] 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 limited in scale.

[7] 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.

[12] During the English Civil War of the 1640s, Calshot was held by Parliamentary forces against King Charles I, and protected with a 15-strong garrison at an annual cost of £107.

[18] In 1804, the castle was used to store munitions for the Sea Fencibles, fishing boats used as volunteer naval vessels to counter the threat of a French invasion.

[19] With the end of the Napoleonic Wars in 1815, the government coastguard took control of the castle, using it as a base to combat smuggling, taking advantage of the water behind the Calshot Spit as a good location to position their waiting interception vessels.

[28] A 1910 plan proposed that the castle would be garrisoned in wartime by 10 officers and 154 men, 75 of whom would have to be housed nearby rather than in the fort itself; additional naval personnel would also have been needed to man the support vessels for the boom.

[35] Calshot twice hosted the Schneider Trophy air races over the Solent, the last two in the sequence of popular international events designed to encourage the development of new, high-speed technology.

[43] After the war, Calshot returned to duty as an active air base, housing two squadrons of Sunderland flying boats which took part in the Berlin airlift of 1948, before being passed across to Maintenance Command in 1953.

[46] Hampshire County Council leased the site from the Crown Estates in 1964; the castle itself passed into the guardianship of the state, and the hangars were used as an activity centre.

[47] A Royal National Lifeboat Institution station opened in 1971 alongside the castle, with a 130-foot (40 m)-tall replacement coastguard tower constructed two years later.

[48] English Heritage took over management of the castle in 1983, and stripped back 20th-century additions to present it as it might have appeared in 1914, including demolishing the old signal station tower.

[53] Calshot Castle is a three-storey, circular fortification, comprising a keep, gatehouse and curtain wall, predominantly constructed of ashlar Portland stone.

[54] When first built in the 16th century, it was designed to carry three tiers of artillery: two positioned on the second floor and the roof of the central keep, and the third in the outer curtain wall.

The castle is surrounded by a water-filled, 16-sided moat, 8.8 metres (29 ft) across, accessed over a 20th-century bridge[51] into the gatehouse, an 18th-century design based on a simpler 16th-century original.

[57] The wall was lowered to its current height in the 1770s and a concrete building to house searchlights, dating from 1896, now stretches along the southern end of the castle.

[57] In the centre of the castle is the keep, which has an external diameter of 16 metres (52 ft), an octagonal lower storey and circular walls on the upper levels.

[58] The second floor was redeveloped in the late 19th century to form another barracks room, with its ceiling incorporating additional early 20th-century girders and concrete to support the gun battery above it.