A royal Commission fort constructed in the 1860s and thought to have been occupied until 1920. World war II concrete additions on top. Built of roughly-hewn stone blocks outside and brick inside. Moat and earthworks on the seaward side. The domestic range along one side of the D-shaped plan is in ruinous condition but the rest of the fort is structurally complete and sound with some noteworthy original iron fittings such as doors and railins. 15 casemates at first floor level with windows nearly all blocked up.