Cromwell's Castle, Tresco
Documents suggest there may have been a small fortification at sea level protecting New Grimsby Harbour in the 16th century and inside Cromwell’s Castle there is a datestone on the inside face of the door on to the gun platform that has the incised legend ‘M1591H’. However, this seems to be a reset stone as its inconspicuous location is at odds with its commemorative contents.
After Tresco was retaken by the forces of Parliament in 1651 a ‘new blockhouse’, Cromwell’s Castle, was built and in 1715 the engineer Christian Lilly described it as: ‘Standing at the Foot of a Steep hill much higher than its Top, and is a Huge Mass of Masonry, consisting of a Round Tower two Storys high, with a Platform for six Gunns upon it, and a Battery before it for Six more at the Watters Edge.’
Cromwell’s Castle is a tall cylindrical building with a lower gun platform attached to its west side. Inside there was a large heated room beneath the vault and in the jambs of the windows there is a wealth of graffiti including some initials that may have been left by soldiers manning the castle in the 18th century. On top there are gun positions all around allowing artillery fire at enemy ships trying to enter the harbour.
For information and a downloadable discussion of Cromwell’s Castle - http://www.english-heritage.org.uk/daysout/properties/cromwells-castle/
For more information on Scilly’s rich military history read Defending Scilly by Mark Bowden and Allan Brodie (English Heritage 2011 £9.99)
http://www.english-heritage.org.uk/publications/defending-scilly/