Cromwell's Castle is an artillery tower, built by Sir Robert Blake following the Parliamentary invasion of the Isles of Scilly in 1651.
During the English Civil War between 1642 and 1646, the inhabitants of the Isles had been Royalist supporters of King Charles I, and rebelled against Parliament in favour of Charles in 1648.
Tresco became a base for Royalist privateers and Parliament became concerned that the Dutch, then hostile to England, might exploit the situation, or that Royalist Irish forces might use the islands for an attack on England.
In 1651 Parliament sent Robert Blake and a naval force to retake the island, which had been fortified by the Royalists.

Having established control of the islands, between 1651 and 1652 Blake constructed Cromwell's Castle on Tresco, named after Oliver Cromwell, the Parliamentary leader.
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.
The Parliamentarian forces were particularly concerned about any potential Dutch attack.

There were two existing fortifications in this location.
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.
There was also a small blockhouse beneath it, dating from the 16th century, and the new castle was built on top of this preexisting site.

The castle initially comprised a three-storey circular tower, 13.45 metres (44.1 ft) across and 15.1 metres (50 ft) high, with walls almost 13 feet (4.0 m) thick, made from massive pieces of rubble; a later survey described the tower as "a Huge Mass of Masonry".
Some of the stone used came from the ruins of King Charles's Castle.
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.
There may have been an adjacent gun platform just beneath the main structure.
The design was old-fashioned, resembling the circular keeps built by Henry VIII in the mid-16th century.
Cromwell's Castle is an artillery tower, built by Sir Robert Blake following the Parliamentary invasion of the Isles of Scilly in 1651. During the English Civil War between 1642 and 1646, the inhabitants of the Isles had been Royalist supporters of King Charles I, and rebelled against Parliament in favour of Charles in 1648. Tresco became a base for Royalist privateers and Parliament became concerned that the Dutch, then hostile to England, might exploit the situation, or that Royalist Irish forces might use the islands for an attack on England. In 1651 Parliament sent Robert Blake and a naval force to retake the island, which had been fortified by the Royalists.
Having established control of the islands, between 1651 and 1652 Blake constructed Cromwell's Castle on Tresco, named after Oliver Cromwell, the Parliamentary leader. 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. The Parliamentarian forces were particularly concerned about any potential Dutch attack.
There were two existing fortifications in this location. 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. There was also a small blockhouse beneath it, dating from the 16th century, and the new castle was built on top of this preexisting site.
The castle initially comprised a three-storey circular tower, 13.45 metres (44.1 ft) across and 15.1 metres (50 ft) high, with walls almost 13 feet (4.0 m) thick, made from massive pieces of rubble; a later survey described the tower as "a Huge Mass of Masonry". Some of the stone used came from the ruins of King Charles's Castle. 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. There may have been an adjacent gun platform just beneath the main structure. The design was old-fashioned, resembling the circular keeps built by Henry VIII in the mid-16th century.
Cromwell's Castle