Spitbank Fort was built following the 1859 Royal Commission. It is one of four "Palmerston Forts" for defending Portsmouth Harbour. It is now a luxury hotel. Viewed here from Southsea's pier. Link
Spit Bank Fort was built in 1867-68, one of 'Palmerston's Follies' - a number of coastal defences constructed in response to France expanding its navy and 'sabre rattling'. In fact the Follies were never challenged hence their sobriquet!