Cromwell's Castle is on the island of Tresco in the Isles of Scilly (grid reference SV882159).
It is a coastal Gun Tower built by Oliver Cromwell in 1651-1652 as a replacement for King Charles's Castle. The guns were mounted on the roof above the garrison's living quarters and magazines. The tower was originally entered at first floor level by an external stair on the south side. The present entrance dates from the construction of the lower gun platform added in the 1740s by Abraham Tovey, Master Gunner.
It is built on a promontory to guard the anchorage between Bryher and Tresco.
It is a Grade II* listed building under the guardianship of English Heritage.