Overlooking the naval base of Portsmouth in Hampshire County, Fort Southwick on the Portsdown Hill stands the highest. The fort acts as the water supplier to the other forts and houses the water storage tanks. The water runs through the brick-lined aqueduct. There is a crescent-shaped barrack block that used to accommodate around 220 men. To defend the dry ditch, there is a full caponier on the north projection along with two other smaller caponiers at the corners. Behind these demi-caponiers, you will find mortar batteries of fiver mortars into the rampart. There is a small musketry gallery that covers a small branch of the ditch.

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