Overlooking Old Grimsby harbour and the anchorage of St Helen's Pool, the blockhouse would have housed a battery of two or three artillery pieces, positioned on a square gun platform on top of a rocky outcrop.

[2] Henry initially responded by fortifying the coasts of England, constructing new artillery forts designed to defend against the longer-range cannons that were becoming common in the 16th century.

[3] As a result of the inspection, Sir Francis Flemming, the Lieutenant-General of the Ordnance, was tasked in February 1558 to improve the defences on the islands, supported by a shipment of lead to help with construction, and money raised from the dissolution of the monasteries in England.

[3] The building work initially focused on the island of Tresco and was carried out under the direction of John Killigrew, the captain of Pendennis Castle in Falmouth.

[6] When complete, it would have held a battery of two to three guns, which could have fired on targets attempting to enter the harbour, or engaged ships approaching the Scilly Isles from the north-east.

[7] Edward Seymour fell from power in 1549, and after his removal a new survey was carried out by Captain William Tyrell, in part to confirm that the new fortifications were suitably located across the islands.

[13] The Old Blockhouse saw military action in 1651 during the interregnum after the end of the English Civil War, making it one of the very few of England's coastal defences to have ever seen actual fighting.

[17] Blake arrived at St Helen's Pool in April 1651 and set about taking the island of Tresco, attacking the harbour of Old Grimsby and the blockhouse.

[22][nb 2] In 1660, Charles II was restored to the throne and Edward Sherburne was sent to the Scilly Isles to inspect the defences; he recommended that repairs be carried out to the Old Blockhouse.

[25] The blockhouse was inspected by Christian Lilly in 1715, on the orders of the Duke of Marlborough; he reported that it was "very much decayed", with only the walls standing, but recommended that it be repaired at a cost of £28.

[6] In the south-west corner of the platform is a lean-to, which may have been either a powder locker or a shelter for the blockhouse watch; a compartment in the south wall might have also been used to store ammunition.