This was an armoured turret built on what was then the end of the Admiralty Pier. This is a unique turret in the United Kingdom, but fairly common for Continental forts.
The turret was built between 1871 and and 1886, after a number of changes in design. It was armed with two 16-inch guns and could be loaded in about 2.5 minutes with the assistance of steam ram and loaders. Each gun wight 80 tons with the whole turret weighing over 700 tons. The magazines were below the guns, close to the waterline.
These type of guns on battleships, were used in the naval bombardment of Alexandria in 1882. The turret never say any action and was regarded as obsolete by 1900.
Between 1909 and 1910 a battery for two 6-inch Mark 7 guns was built on top of the original fort.