Unlike its big brother, Hurst Castle just down the coast, Calshot Castle was never that strategically important. Originally built in 1540 as one of the Device Forts, the exterior is largely unaltered.
The only significant alterations are inside, where a massive concrete floor was inserted over the basement when it was adapted to house a generator for the searchlights that were installed in 1896.
Check the English Heritage website for opening times etc.
Nearby is the Calshot Activities Centre housed in the old flying boat hangers. The bar/refreshment room in the main hanger does a nice line in snacks and has wonderful views across the Solent.