Built between 1861 to 1878,Spitbank Fort lies off the coast near Southsea.Spitbank is smaller than the two main Solent forts, Horse Sand Fort and No Man's Land Fort. Its main purpose was as a further line of defence for ships that made it past the two main forts. It is 49.4 metres (162 ft) in diameter across at its base, with one floor and a basement and armour plating only on the seaward side. It was originally planned to have been armed with nine 10" eighteen ton rifled muzzle loader (RML) guns on the seaward side, and six 7" seven ton RML guns on the landward side. However, by the time of completion the plan had changed so that the seaward side received nine 12.5-inch muzzle-loading (RML) guns. From 1884 more modern 12-inch breechloading guns were installed and these were in service until after World War I.