Calshot Castle

Henry VIII's 1533 break with Rome over the annulment of his marriage to Catherine of Aragon offended her nephew Charles V; France and the Empire allied against Henry in 1538 and invasion looked certain. Henry responded with a "device" in 1539 ordering forts built along the coast. Calshot was one of four that Lord High Admiral William Fitzwilliam and William Paulet raised along the Solent to defend Southampton Water, alongside East and West Cowes and Hurst.

Built on the narrow Calshot Spit above the deep-water channel into Southampton, the castle followed temporary earthworks and was finished by the end of 1540, probably using stone and lead salvaged from the dissolved abbeys at Beaulieu and Netley. Its first garrison was eight gunners, five soldiers and a lieutenant under a captain; by the late 1540s it carried up to 36 guns.

A fire in the 1580s required 127 trees from the New Forest for repairs, carried out in 1584 amid fear of Spanish invasion, by which point the garrison had shrunk to eight men. During the Civil War of the 1640s Calshot was held by Parliamentary forces against Charles I with a fifteen-strong garrison; Parliament kept it supplied and operational, probably because of its ongoing role defending Southampton Water. It carried up to 25 guns during the early eighteenth-century War of the Spanish Succession.

The castle was modernised in the 1770s with a new parapet, altered gun embrasures and an enlarged gatehouse. An inspection in 1793 found its guns in poor condition under an "infirm, elderly master-gunner"; by 1809 the geographer James Playfair still described Calshot as merely "a blockhouse with a garrison". In 1804 it stored munitions for the Sea Fencibles, volunteer fishing boats raised against French invasion.

After the Napoleonic Wars ended in 1815, the coastguard took over Calshot as a base against smuggling. Renewed military interest in the 1850s produced proposals to arm it with up to 32 guns, not carried out; it was noted that the sixteenth-century keep would throw dangerous stone splinters if struck by enemy artillery.

Concern in the 1880s that Southampton was vulnerable to French torpedo-boat attack led the War Office to reclaim the castle from the coastguard in 1894 and build a boom across Southampton Water. The old castle was too small to mount a battery of its own, so a larger one was built south of it in 1895, backed by searchlights; the boom was worked from two towers called "dolphins", each with quick-firing guns.

A Royal Naval Air Station was built beside the castle in 1913 to house twelve experimental seaplanes supporting the Channel fleet. When the First World War broke out the following year, defensive thinking shifted to gun batteries at the mouths of the Solent; the boom was removed and replaced by anti-submarine nets further up the coast, and two of the castle's guns were taken in 1915 to protect them. The airbase trained pilots until 1916, then flew anti-submarine patrols, logging more than 9,000 hours of flying in just three months of 1918. Calshot's remaining guns were removed, probably sent to the front in France.

As RAF Calshot between the wars, the station ran the School for Naval Co-operation and Aerial Navigation from 1918 and took on the Seaplane Training Squadron in 1931; the 1895 battery was demolished for space. It hosted the last two Schneider Trophy air races over the Solent.

At the outbreak of the Second World War, Calshot was defended by troops of the Hampshire Regiment and a barge carrying anti-aircraft guns. Five of its boats took part in the Dunkirk evacuation, and as invasion fears grew, two guns and searchlights were added to the keep's roof in 1940. The castle itself was not damaged, and by 1943 it was on a "care and maintenance" footing as a stopover for passing aircraft.

After the war Calshot resumed active service, and two squadrons of Sunderland flying boats based there took part in the Berlin airlift of 1948. A signal station with radar and radio, built on top of the keep from 1953, opened in 1958, but seaplanes were by then obsolete and RAF Calshot closed in 1961.

Hampshire County Council leased the site in 1964 and the castle passed into state guardianship. English Heritage took over its management in 1983, stripping later additions to present it as it appeared before 1914. It is protected as a Grade II* listed building and a scheduled monument, remains open to visitors, and its coastal position leaves it exposed to erosion and rising sea levels.