Calshot Castle is an artillery fort constructed by Henry VIII on the Calshot Spit, Hampshire, England, between 1539 and 1540. 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. The castle had a keep at its centre, surrounded by a curtain wall and a moat. Initially heavily armed, it had a garrison of 16 men and as many as 36 artillery guns. The castle continued in use for many years, surviving the English Civil War intact and being extensively modernised in the 1770s. During the 19th century, Calshot Castle was used by the coastguard as a base for combating smuggling. In 1894, however, fresh fears of a French invasion led to it being brought back into use as an artillery fort: a large coastal battery was constructed alongside the older castle and a boom built across Southampton Water, controlled from the castle. During the First World War, Calshot Castle was primarily used as a base for seaplanes, deployed on anti-submarine patrols in the English Channel; its guns were removed before the end of the war, probably for use in France. The air base, by then called RAF Calshot, grew in size during the inter-war years, hosting the Schneider Trophy air races. With the outbreak of the Second World War, Calshot was re-armed in the face of a possible German invasion. The station continued in use after the war, but as military seaplanes became obsolete, it was finally closed in 1961. After a short period of use by the coastguard, the castle was opened to the public by English Heritage in the 1980s. Restored to its pre-1914 appearance, the castle received 5,751 visitors in 2010. Historic England considers Calshot a "well-preserved example" of King Henry's Device Forts. History 16th century Construction 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. Modest defences based around simple blockhouses and towers existed in the south-west and along the Sussex coast, with a