Horse Sand Fort is one of the Solent Forts recommended by the Royal Commission in 1860. It was built between 1865 and 1880 at a cost of £424,694, a huge amount of money at that time. It has two floors with compound armour plate, twenty five inches thick, shielding the guns. The magazines were in the basement under the sea level.
The original armament was intended to be twenty-four 10-inch RML guns on the lower floor and twenty-five 19-inch RML guns on the upper deck. There was plans for five turrets on the roof, each holding two 12.5-inch RML guns but these turrets were never built. It is doubtful the Fort was ever fully armed as planned. By 1871 the armament had been upgraded on paper to be armed with thirty-four 9-inch RML guns and some fifteen 12-inch RML guns. This was further amended four years later to twenty-four 12.5-inch RMLs on the lower level and twenty-five 10-inch RMLs guns on the upper level.
By 1883 a new proposal was made for seven 12-inch Mark I BL guns to be added on each level in the sea facing casemates. This was further amended in 1883 to ten of the 12-inch BL Mark I guns to be mounted with ‘Yoke’ mounting. The 12-inch guns were identical to those installed on No Man Land Fort. In addition some twenty-four 6-pdr Q.F. guns were to be added. These 12-inch guns and 6-pdr Q.F. guns were emplaced by 1893.
In 1899 three 6-inch BL Mark VII guns were proposed to be mounted on the top deck of Horse Sand Fort, and one was in place by 1904. However, the restricted field of fire in this enclosed spaced made it unsuitable and in 1908 the three 6-inch BL guns had been mounted on the roof.
In 1908 to 1909 a submerged concrete barrier was built under the sea running towards Southsea Castle. There was also a shorter leg running towards No Man’s Land Fort. This barrier was to deter larger vessels from running past the Fort, in effect a submerged boom defence.
During World War 2, a pair of 6-inch BL guns were mounted on the roof of Horse Sand Fort in the old emplacements with new brick overhead cover.