Fort Southwick

Fort Southwick stands on Portsdown Hill above Portsmouth, a 19th-century fort built to protect the naval base. It is the highest of the hill’s forts and once supplied water to the others through a brick-lined aqueduct. Construction began in 1861 and took nine years to complete, designed to house around 220 soldiers in a crescent-shaped barrack block. The fort’s defences included caponiers, demi-caponiers and mortar batteries, with 23 guns installed by 1893. A spiral staircase led to tunnels that connected the barrack block, caponiers and magazine. The fort played a key role in the Second World War when 172 Tunnelling Company dug a secret underground headquarters beneath it. This network of tunnels served as the communications centre for Operation Overlord under the call sign ‘MIN’. After the war it remained in military use, first as a barracks, then as a demobilisation centre and later part of the Admiralty Research Establishment. The site was sold in 2003 and now houses NHS facilities alongside a park and ride service. Visitors find a Grade I listed fort with well-preserved 19th-century defences and wartime tunnels. The barrack block survives intact, and the tunnels beneath offer a glimpse into the fort’s secret role in the war. The surrounding hilltop setting provides views over Portsmouth and the naval base. The site remains active as part of the NHS, blending military history with modern use.