The five forts, with two supporting batteries, were built between 1861 and 1874 on the top of Portsdown Hill overlooking Portsmouth to protect the harbour from an invasion force attacking from the north or east by land. The five Gosport Forts protected the west side of the harbour. They have a low profile and are disguised by earth and grass banks. A six sided design was used for Fort Nelson (with 26 guns) and Fort Wallington (17 guns) and seven sided for the remainder - Fort Purbrook, Fort Widley (21 guns) and Fort Southwick (23 guns). All had a complement of about 220 men. Fort Purbrook had two additional defenses to protect its eastern side, Farlington Redoubt (demolished in 1970) and Crookhorn Retreat (destroyed by 1876).
Fort Purbrook is now an activity centre, Fort Southwick remains MOD property and Fort Wallington is mainly demolished and is now part of an industrial estate. Fort Nelson is now a military museum, open April - October.