Harry's Walls

 military, ruins, fortification

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Harry's Walls comprise of an unfinished artillery castle dating to the mid-16th century, situated on the summit of Mount Flagon, overlooking St Mary's Pool.

It was begun in 1551 as part of a major phase of fortification on the Isles of Scilly, undertaken between 1547 and 1554, to counter threats from the French. By 1554, some of the garrison was transferred to Tresco, reflecting a shift in the focus of fortification.

This artillery castle would have been the first fortification in the country to have been provided entirely with angled bastions and straight orillions, a design at the forefront of military engineering. However it was never completed since the site was not entirely suitable. The crown of Mount Flagon was too small to accommodate a castle of this size and the location was strategically poor, unable to fully cover the approaches to the anchorage.

On the south west side of the monument is a massive stone curtain wall terminating in a pointed bastion at each end, facing west and south. On the north west side is an unfinished rock-cut ditch. The curtain wall is faced on each side by granite blocks and is infilled with granite rubble. The bastion walls, either side of the curtain wall, are defined by massive acutely angled walls pointing directing away from the corner of the castle. They average 21 metres long and 5 metres wide and rise up to 2.3 metres high externally.

The angled outer walls of each bastion are joined to the ends of the curtain wall by a short linking wall. These were designed to mount guns that would provide flanking fire along the outside of the curtain walls. Within each bastion is a small sub-triangular internal area linked to the castle interior by a narrow entry passage through the walls.