Spitbank Fort is on the four sea forts, protecting the navigable channels leading into Portsmouth Harbour. Spitbank Fort lies close to the entrance and is smaller than No Mans Land and Horse Sand Fort, but is similar in dimension to St Helen’s Fort. Spitbank Fort only has armour on half of its’ gun deck, the other have being granite. The armour is some twenty five inches thick made up of layers of iron plates, wrought-iron and teak.
Spitbank Fort was built between 1867 and 1878 with the armoured half of the fort, which was facing the open sea, armed with nine 10-inch RML guns. The landward facing casemates, made of brick and faced with granite, mounted six 7-inch RML guns. The two iron turrets with 12.5-inch RML guns planned to be mounted on the roof, were never built. The Fort also had two Defence Electric Lights.
By 1886 the 10-inch RML guns had been replaced by nine 12.5-inch RML guns.
In 1889 two 4.7-inch Q.F. guns were mounted on the roof of the Fort. The 7-inch RML guns appear to have been removed at this time. By 1906 the number of 12.5-inch RMLs had been reduced, and these were probably just to cover minefields and the submarine defence line.
In 1907 the three 12.5-inch RML were removed, leaving just the two 4.7-inch Q.F. guns. In 1909 a pair of 6-inch BL guns were added along side the 4.7-inch guns. This configuration of four BL guns on the roof continued until 1917.
The YouTube Video by David Moore give great view of the casemates and roof top guns. See https://www.youtube.com/watch?v=t-ReKOD7otA.