King Charles's Castle is a coastal artillery fort near the northern extremity of the island of Tresco in the Isles of Scilly. It is now in ruins and the site is administered by English Heritage. It was a semi-hexagonal structure to provide a wide field of fire and two-storeyed to give at least two tiers of guns. The domestic quarters for the garrison were at the rear.
Despite its name, the castle was built 1550-1554 during the reign of Edward VI. During the Civil War low earthwork defences were thrown up beyond the castle to protect it from landward attack. After the war it was replaced by Cromwell's Castle.