17th-century round tower positioned on a rocky promontory guarding the anchorage between Bryher and Tresco.
Small but mighty, Cromwell's Castle guards the approach to Old Grimsby harbour on Tresco. The English Heritage-managed fortress stands on a low, rocky promontory, and once provided a formidable deterrent to any approaching enemies: how could they get past its batteries?
The castle partly owes its excellent strategic position to the failure of an earlier fort. King Charles's Castle is situated higher up the hill, and its elevation meant that its guns had to be angled downwards towards invading vessels (the cannonballs rolled out before they could be fired). When Parliamentarian troops overthrew the defending Royalists, they built this new fortress to defend the island.
This makes Cromwell's Castle pretty unique: there isn't much surviving Interregnum military architecture. In an act that's simply pragmatic but seems like one upmanship, stones from King Charles's Castle were used to construct the new fort in 1651. A good way to see the relative merits of the forts' positions is to take the six-mile Tresco coastal walk recommended by English Heritage, which takes in both castles plus a blockhouse.
Today, you can climb up the remarkably intact two-storey tower and stand on the gun battery on the roof. There's also a large 18th-century gun platform next to the tower, as well as a guardroom and latrine from this period. Like the other English Heritage defence sites on Scilly, Cromwell's Castle is free to visit and open during daylight hours.