Originally designed for two tiers of guns mounted in a circle, with a boom strung between them, there were many problems with subsidence, and after extensive cost overruns the forts were completed in 1871 with one tier, and 11 guns : a mixture of 9-inch and 7-inch rifled muzzle-loaders, and no boom. The forts were never used in anger, and were disarmed before the First World War.