King Charles' Castle

King Charles's Castle is a ruined artillery fort overlooking New Grimsby harbour on the island of Tresco in the Isles of Scilly. Built between 1548 and 1551 to protect the islands from French attack, it would have held a battery of guns and an accompanying garrison, designed to prevent enemy vessels from entering the harbour. The castle is polygonal in design, constructed from granite stone, with the gun battery at the front, and a dining room, kitchen and living accommodation at the rear. An additional defensive earthwork was constructed around it during the 17th century. The design of the castle is unusual for the period, and is only seen elsewhere in blockhouses along the River Thames.

The castle's design was unsatisfactory, as its guns could not be angled so to fire down into the harbour, and its defences were considered vulnerable to attack.