The strategic importance of the channel between Tresco and Bryher in theIsles of Scilly in the defence of the British Isles has been recognised across the centuries. In 1550, during the reign of Edward VI a small artillery fortress was built on the high ground of Tresco. Its purpose was to defend the harbour of New Grimsby. At the beginning of the Civil War it was garrisoned by Royalist troops. It later became known as King Charles’s Castle.
However, the site of the fortress was considered unsuitable and when the Parliamentarians gained power a new artillery tower was built on the rocky shore in 1651. Its purpose was to defend against all threats – notably, at that time, from the Dutch.
(See also: https://lagill6.wordpress.com/2018/11/01/room-with-a-view/)