 After 1890 the Citadel ceased to have a defensive role and functioned instead as a large barracks and mobilisation centre, with additional hut accommodation provided for up to 900 soldiers in total.In 1956 the Citadel was handed over to HM Prison Service; it served successively as a Borstal, a Youth Custody Centre and a Young Offender Institution.It was initially envisaged that the Citadel would be armed with forty-three 18-pounder guns, and thirty-one carronades.