Fort Southwick is one of the forts found on Portsdown Hill, which overlooks the naval base of Portsmouth in the county of Hampshire, England. It is the highest fort on the hill, and holds the water storage tanks for the other forts, supplying them via a brick lined aqueduct. Construction was started in 1861 and completed by 1870. It was designed to house a large complement of men (about 220) in a crescent-shaped barrack block. Its north projection has one full caponier to defend the dry ditch, and has two smaller demi-caponiers at the corners. A small musketry gallery crosses the ditch at the south-west angle to cover a minor branch of the ditch.