1841-6 defensible barracks, built to house officers and men of the Royal Marines, and, to a limited extent, to provide landward defence for the Royal Dockyard.
The centre of the platform is a 140 foot (42.67m) square parade enclosed by barrack blocks, entered via a projecting N gatehouse. Coursed rubble stone with ashlar dressings, band, parapet and hipped slate roofs with red brick chimneys. The gatehouse has a pedimented ashlar front, five musketry loopholes over a sill band and moulded cambered arched gateway with datestone 'VE 1844' over and two flanking loopholes. Wall breaks back each side with loophole each floor. Winches for the former sliding drawbridge are in the rooms each side of the entry. The flank walls of the gatehouse are loopholed on both floors and return to the N side of the barracks. The barracks are four two-storey ranges with basements around the central square, the corners chamfered for access passageways to the bastions through tall cambered archways with double wooden doors. The external elevations were originally pierced with 419 ashlar square musketry loops (the number since reduced by the insertion of some sash windows), in two floors and arranged in groups. There is a central projecting square block to the W, E and S sides with sash windows to the front and loops to the sides, the E block carrying an iron water tank. The chamfered corners have steps down to basement stores each side of the gateways, four loops each side at ground floor and eleven at first floor. The N external wall has contemporary brick additions (marked on 1862 plan), to the W of the gatehouse with flat concrete roof, to the E with hipped slate roof, a former detention room and cell block. A small structure to the E was a fire-engine shed. Later C19 additions on the S wall, a 5-bay lean-to timber gun shed, a gabled billiard room and latrine. The W wall formerly had a lean-to timber 'skidding shed'.
The Courtyard Elevations of Barracks: A fine Georgian-style square with pedimented centrepieces on three sides. 12-pane sash windows in flush surrounds, plain band and parapet. Continuous basement area crossed by ashlar steps on brick half-arches and protected by cast-iron rails with Prince of Wales feathers and VR crests.
W and E sides match, 5-3-5 bays with centre doorway, the N side is of 5-2-1-2-5 bays, the centre having the main gateway with sash above and pediment with clock. The doorways are in the centres of the two 5-bay sections. The S side is of 5-9-5 bays, the centre slightly recessed, one doorway in each of the outer sections and three in the 9-bay centre, each alternate doorway with a later timber shallow-pedimented porch. All the doorways have shallow-gabled flush ashlar surrounds.
Scheduled Ancient Monument Pe 379.