Calshot Castle was built in 1539-40, on the end of a short shingle spit, to guard the entrance to Southampton Water. It formed part of Henry VIII's chain of coastal defences to counter the threat of invasion by the Catholic powers of Europe. In 1913 the castle became the nucleus of one of the first Royal Naval Air Stations. In 1918 it was renamed RAF Calshot. From 1945 it was a coastguard station until it was handed over to English Heritage.