Old Blockhouse

History of Old Blockhouse

This small blockhouse was built between 1548 and 1554 to defend the harbour of Old Grimsby, on the east coast of Tresco.

The Old Blockhouse built on a rocky outcrop with the sea in the background
The Old Blockhouse

The Block House formed part of a series of forts built on the islands during the reigns of Edward VI and Mary I, and followed the attempt to build a large artillery fort, known as Harry’s Walls, on St Mary’s.

In fact, the blockhouse did not see action until the 17th century, during the English Civil War. In 1651 a Royalist garrison was defending the islands against attack by a Parliamentarian fleet commanded by the famous Admiral Blake.

Blake attacked Tresco first, capturing its fortifications without difficulty, and then used the island as a base to assault the main Royalist stronghold at Star Castle on St Mary’s. Though vigorously defended, the Old Blockhouse clearly proved of little value against the longer-range guns of Blake’s ships.

It was replaced afterwards by the much stronger gun fort, called Cromwell’s Castle, on the other side of the island, though it was still considered serviceable in the 18th century.

Looking through an opening out to sea across the gun platform
Looking out to sea across the gun platform


Blockhouses are small, strongly-built defensive structures, often constructed in forward positions. Designed to house guns and protect gunners and their ammunition from attack, they were used from the late-14th to mid-17th centuries.

The Old Blockhouse sits at the south-east edge of Old Grimsby harbour, from where it commands a field of fire to the north-west, across the entrance to the harbour, and to the north-east, across the waters between Tresco and Tean, one of several points of entry into the Scilly archipelago.

The simple structure here consists of a rectangular gun platform, 23 ft (7m) by 20 ft (6m), built on a natural rocky outcrop and approached by a flight of seven steps. It was once surmounted by a parapet with splayed gunports; although now partly demolished, the parapet remains up to 3 ft (1m) thick in parts.

A small lean-to in the south-west corner was designed to accommodate the handful of men charged with keeping watch. In the south wall there is a small recess which may have been an ammunition store. To the right of the entrance is a second room, with a fireplace and chimney, which was probably the later living quarters, added when the blockhouse was reoccupied during the Civil War.

The approach to the Old Blockhouse is defended by a U-shaped rampart, visible as a turf-covered bank up to 33 ft (10m) wide and 6 ft (2m) high, around the crown of the hill, while two lower ramparts survive lower down, covered in blown sand.

The gun platform with lean-to shelter
The gun platform with lean-to shelter

Further Reading

Bowley, RL, Scilly at War (St Mary’s, Scilly Isles, 2001)

Morley, B, Henry VIII and the Development of Coastal Defence (HMSO, London, 1976)

Tolhurst, M, The English Civil War (English Heritage, London, 1992)


The text on this page is derived from the Heritage Unlocked series of guidebooks, published in 2002–6. We intend to update and enhance the content as soon as possible to provide more information on the property and its history.

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