Fort Southwick was built to defend the landward approaches to the naval base on the recommendation of the Royal Commission on the Defence of the United Kingdom which reported in 1860.

In 1893 the fort was armed with a total of 23 guns: A central spiral staircase from the surface gives access to four main tunnels running radially outward from it at uneven angles.

During the Second World War, Underground Headquarters of UGHQ, the communications "nerve centre" for Operation Overlord, was in a secret network of tunnels excavated between February and December 1942 by 172 Tunnelling Company of the Royal Engineers 100 ft (30 m) underneath Fort Southwick, well out of reach of any bombs of the era.

It was sold by the Ministry of Defence in July 2003 to the "Fort Southwick Company Limited", who intend to convert the barrack block into luxury apartments.

[1] The site is currently occupied by the NHS and includes a staff park and ride service to Queen Alexandra Hospital.