It became clear that Tresco needed to be defended, as the Dutch also posed a threat to the security of Scilly. The advantage that invading forces would find in Tresco harbour was that It was very sheltered (by Bryher to the west and Tresco to the east) and therefore an extremely good place to moor invading ships. With the harbour covered, ships would be forced to moor in other, more exposed areas.

Cromwell’s? Castle was built using some of the stones from the earlier King Charles? castle (probably the most important and useful role of Charles? castle!).

Cromwell’s castle protected Tresco harbour from the north and from the south and as it was closer to the sea, canon fire would have been much more effective than King Charles?.