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.