St Mawes Castle, in fact. Built (together with Pendennis Castle on the other side of the Carrick Roads - which is actually an inlet of the Channel) by Henry VII. It was a Royalist stronghold in the Civil War, but was taken quite easily by Cromwell (cough, spit) because its guns could not be traversed far enough to repel a landward attacker. (Presumably, Henry VII expected only to be attacked by foreigners.)