St Helens Fort was built between 1865 and 1878 and cost £123,311. It faced many problems during construction which result in it mounting only three guns on the roof. Two 10-inch RML gun on the flanks and a single 12.5-inch RML in the centre facing across the approach channel to Portsmouth. This fort is the same size as Spitbank For, is smaller than the other two Solent Sea Forts, Horse Sand and No Man’s Land.
It is also the only one of the sea forts that has the artillery concentrated in emplacements on the roof. The 12.5-inch is in a shielded emplacement with its’ movement restricted by the arc of the racers. The two 10-inch guns, one on each side, are mounted in granite masonry but shielded by iron. The guns are mounted on a moveable table which allows fire through two gun ports. The turntable is powered by steam which would have assisted in moving the gun into the correct position to engage. The design of this turntable is very similar to that used for the four 12.5-inch guns mounted in similar design positions in Gibraltar. These batteries were King’s Bastion, Princess Alexandra Battery, Wellington Front and Zoca Flank which had their 12.5-inch RML guns mounted behind a curved iron shield with two apertures for firing. The gun was also moved on a turntable powered by steam. These turrets were designed by Colonel Jervois who was also involved in the building of the Solent Fort.
