Strategically sited to command an uninterrupted field of fire over the entrance to Falmouth Bay in tandem with Pendennis Castle, on the opposite side of the estuary, St Mawes is one of the network of 16th-century coastal fortresses built by Henry VIII, and is also among the best preserved. Cloverleaf shaped, with circular towers around a central keep, it's approached via a classic drawbridge, and you can wander freely around the interior chambers and the outside gun decks.
Ironically, the castle never really saw much action; the threat of Catholic invasion from Spain didn't materialise during Henry VIII's reign, and it was easily taken by Parliamentarian forces during the Civil War. Still, it's an atmospheric place to explore and let your imagination roam.