A Royal National Lifeboat Institution station opened in 1971 alongside the castle, with a -tall replacement coastguard tower constructed two years later. English Heritage took over management of the castle in 1983, and stripped back 20th-century additions to present it as it might have appeared in 1914, including demolishing the old signal station tower. The site remains open to visitors and received 5,751 visitors in 2010. The castle is protected under UK law as a Grade II* listed building and as a scheduled monument, but its coastal position means it is at risk from erosion or changing sea levels caused by climate change. The air station's former hangars are still used as a recreation centre by Hampshire County Council. Architecture Calshot Castle is a three-storey, circular fortification, comprising a keep, gatehouse and curtain wall, predominantly constructed of ashlar Portland stone. When first built in the 16th century, it was designed to carry three tiers of artillery: two positioned on the second floor and the roof of the central keep, and the third in the outer curtain wall. Additional guns could have been placed on the roof of the gatehouse and on the first floor of the keep. Historic England considers it to form a "well preserved example" of the Device Forts. The castle is surrounded by a water-filled, 16-sided moat, across, accessed over a 20th-century bridge into the gatehouse, an 18th-century design based on a simpler 16th-century original. The gatehouse was altered in 1896, with the addition of brick-built ancillary buildings to the southern end. It was probably intended to provide additional living space for the garrison. The gatehouse leads into what was originally a 16-sided courtyard with 15 gun embrasures round the curtain wall. The wall was lowered to its current height in the 1770s and a concrete building to house searchlights, dating from 1896, now stretches along the southern end of the castle. In the centre of the castle is the keep, which has an external diameter of , an octagonal lower storey and circular walls on the upper levels. Originally this held accommodation for the captain and the garrison, but it was heavily redeveloped in the 19th and 20th centuries. Its exterior walls have eight recesses at ground level, originally used to store ammunition for the castle's guns. The basement of the keep was redesigned in the 1890s, when electrical generators were installed, protected by a new,