[4] It was intended to protect the deep water entrance to New Grimsby harbour on the west side of the island, a route which could also allow enemy vessels access through to the other Scilly Isles.

[7] The main existing fort, King Charles's Castle, had been built in the 1550s but was poorly sited and had been blown up by its defenders when Blake's forces took the island.

[11] Six gun-ports with broad external splays on the roof allowed the battery a good angle of fire across the channel between the islands of Bryher and Tresco.

[16] Abraham Tovey, the Master Gunner at the nearby fortification known as the Garrison, constructed a large gun-platform for a battery of six guns on the south-west side of the tower, possibly on top of any pre-existing 16th-century platform.

[19] The writer John Troutbeck, commenting at the end of the century, remarked on the tower having a "special bomb proof" roof, topped with 6-foot (1.8 m) high parapets.

[23] English Heritage have commented that the castle is "one of only a few stone fortifications that survive from the Interregnum", and it is protected by UK law as a Grade II* listed building.