The redoubt is a circular structure, measuring 224 feet (68 metres) in diameter and is built almost entirely of brick with some granite facing. The lower tier is composed of a ring of 24 casemates or vaulted chambers, which open into a central parade ground. Casemate 11 was the main magazine for the redoubt, casemate 8 was modified in the 1870s as a cook house and casemates 23 and 24 were altered in the 1880s to provide a detention room and two cells.