The Scilly Isles supported Charles I during the English Civil War, and after a short period in Parliamentary control rebelled in favour of Charles in 1648.; The Royalists on Tresco appeared to have believed that King Charles's Castle was a critical defensive point on the island and probably built new earthwork defences around the castle to defend it against an attack from the land. It is possible, however, that these defences instead date from around 1627, and were built by the King's engineer, Bernard Johnson.; During this period the castle was given its current name.
