A Henrician artillery castle built in 1539-40 by Henry VIII as part of his network of coastal defences defending against the threat of French and Spanish attacks. The castle was built to defend the entrance to Southampton Water and was part of the Solent defences along with East and West Cowes and Hurst Castle. The castle was altered various times in circa 1770, 1868-71 and in the 20th century. Three searchlights (Defence Electric Lights) were constructed circa 1896 and a coastal battery was built in 1902. The castle became part of RAF Calshot in the First and Second World Wars and continued to be used as a seaplane base until it closed in 1961. The coastal battery was re-equipped in the Second World War as part of the Southampton Water coastal defence system. Calshot Castle was restored by the Department of Environment in 1985.
The stone built castle consists of a three-storey central tower surrounded by a lower concentric curtain wall and a moat. There were no outer bastions. It was equipped with eight embrasures and 18 gun-ports on three tiers of the tower and on the curtain wall. The gatehouse had a further three guns to defend the castle from being attacked along the spit it was built on. Entrance to the fort was across a drawbridge on the south-east side and through the two storey gatehouse.
In 1902 a coastal battery was constructed at the castle and armed with two 4.7 inch Quick-firing (QF) and four 12 pounder QF guns. These were changed in the Second World War to two 12 pounder QF guns.