The castle was captured by the rebels during the Prayer Book Rebellion of 1549, and was garrisoned by the Royalists during the English Civil War in the 17th century. Towards the end of the civil war it was stripped for its building materials and rendered largely uninhabitable. A small gaol was erected in the centre of the bailey, which was also used for executions. The castle eventually became the county gaol for Cornwall, but was heavily criticised for its poor facilities and treatment of inmates. By 1842, the remaining prisoners had been moved to Bodmin Gaol and the site was closed, the castle being landscaped to form a park by the Duke of Northumberland. During the Second World War, the site was used to host United States Army soldiers and, later, by the Air Ministry for offices. The ministry left the castle in 1956 and the site was reopened to visitors.