Paget Island had been used to mount coastal defence guns since at least 1612, but subsequent batteries had been lost to the sea. In 1740 the Fort mounted 10 guns and was used to hold prisoners of war. This Fort was badly damaged in a storm of 1791 and the old Fort Paget was finally replaced by Fort Cunningham in 1875. The Fort was named after Captain Thomas Cunningham of the Royal Engineers who supervised the building of the Fort.
The Fort was armed with seven RML guns including two 38 ton, 12.5-inch guns. These guns were mounted behind 15-inch iron plates, reinforced by 20-inches of teak wood. Unlike many shielded positions built around the World, where each gun had its’ own shield set in granite, here five of the guns were behind a continuous iron shield. This was the same as the Solent Sea Forts, but not normally seen in Forts built on land.
Two 6-inch guns were subsequently built on the ramparts of the fort to replace the redundant RMLs.