There are three monuments in State Guardianship on Tresco in the Isles of Scilly. Cromwell’s Castle, King Charles’s Castle and the Old Blockhouse. All three were acquired in 1950 (and feature in the Ministry’s guidebook).
The Old Blockhouse at Old Grimsby and King Charles’s Castle (note the sign uses King Charles’ Castle) near New Grimsby are contemporary.
The text for the sign echoes Bryan H. St John O’Neil’s guide:
Guidebook: ‘The western end is semi-hexagonal in order to provide a wide field of fire, and was two-storeyed to give at least two tiers of guns.’
Sign: ‘The western end was semi-hexagonal to provide a wide field of fire and was two-storeyed to give at least two tiers of guns.’
Guidebook: ‘… one bastion and a demi-bastion … It was intended to protect the castle from a landward attack across the headland.’
Sign: ‘During the Civil War, low, earthwork defences of bastioned form were thrown up beyond the castle to protect it from landward attack.’
I am grateful to Patrick Taylor for digitising images of these signs.