Old Blockhouse

Visitor Guide

The Old Blockhouse is reached along an unsurfaced coastal footpath from Old Grimsby, Tresco's smaller and quieter harbour settlement on the island's eastern side. As with everywhere on Tresco, there is no vehicle access, so the walk is the only way in, though the path itself is considerably gentler than the steep climbs required to reach King Charles's Castle or Cromwell's Castle elsewhere on the island.

The surviving ruins are modest: low, thick stone walls that once formed a compact gun tower commanding the harbour entrance below. What the site lacks in scale it makes up for in atmosphere and setting, with fine views out across Old Grimsby Sound towards the neighbouring island of St Martin's, and a genuine sense of quiet remoteness that contrasts with the more visited forts on Tresco's western side.

Allow around fifteen to twenty minutes to explore the site itself, though many visitors combine it with a longer coastal walk around this less-trodden side of Tresco. A shop and cafe in Old Grimsby offer refreshments, and the walk can easily be extended towards Tresco Abbey Garden further south for a fuller day out on the island.

History

The Old Blockhouse was built between 1548 and 1554, during the reign of Edward VI, as part of a wider mid-Tudor programme to strengthen coastal defences across the Isles of Scilly and the south-west more generally, in response to persistent fears of French and Spanish naval aggression. It was one of several small fortifications raised on Tresco during this period, positioned to command the entrance to Old Grimsby Harbour, an anchorage of real local importance on the island's more sheltered eastern coastline.

Its design was modest by comparison with the larger works being undertaken elsewhere in the islands at the same time, notably the arrow-headed bastions of Harry's Walls on St Mary's, but it served its intended purpose as a visible deterrent and a base for a small garrison able to respond to shipping movements in and out of the harbour. For roughly a century after its construction, the blockhouse appears to have seen relatively little direct action, its presence more preventative than combative, a common fate for many of the smaller Tudor coastal defences built during this era of intermittent rather than sustained threat.

That changed in 1651, during the closing stages of the English Civil War. Admiral Robert Blake's Parliamentarian fleet used Tresco, including the shelter of Old Grimsby and New Grimsby harbours, as a staging ground for the assault that ultimately forced the surrender of Royalist forces holding out at Star Castle on St Mary's. Although the decisive engagements of that campaign centred on St Mary's itself, Tresco's harbours and fortifications, the Old Blockhouse among them, played a supporting role in securing the island as a base of operations during this final chapter of the war in the south-west.

In the aftermath of 1651, Cromwell's Castle was built on the opposite, western side of Tresco to guard the more strategically significant New Grimsby Sound, and it was this newer fortification that took over as the island's principal defensive work. The Old Blockhouse, however, was not abandoned outright: records suggest it remained in reasonable repair and was still regarded as a serviceable defensive position well into the 18th century, a testament to the durability of its original Tudor construction. Today the ruins are cared for by English Heritage, a quieter but genuinely rewarding counterpart to Tresco's better-known fortifications.

What survives today is modest in scale compared with the more substantial ruins of Cromwell's Castle or the extensive earthworks of Star Castle on St Mary's, but the Old Blockhouse's longevity is itself part of its story. Few of the smaller Tudor coastal defences built across the south-west in the mid-16th century remained in any kind of active use for as long as two centuries, and its continued serviceability into the Georgian era says as much about the soundness of its original construction as it does about the enduring strategic value of Old Grimsby Harbour as a sheltered anchorage on Tresco's more exposed eastern coastline.

Important Features

• The gun tower ruins: low, thick stone walls surviving from the original 1548–54 construction.
• Its harbour-side position: built to command the entrance to Old Grimsby Harbour.
• Views over Old Grimsby Sound: outlooks across to the neighbouring island of St Martin's.
• Its role in 1651: part of the Parliamentarian staging ground during the recapture of Scilly.
• Long working life: still considered a serviceable defensive position into the 18th century.
• Its quiet setting: among the least visited of Tresco's fortified sites, with a strong sense of remoteness.

Visitor Information

Things to Do Nearby

Frequently Asked Questions

Official English Heritage Site

For the latest access information, events and any temporary restrictions, check the official English Heritage page for the Old Blockhouse before you travel.

Visit the official English Heritage website