16th-century fort built over St Mary's Pool harbour, but never finished
The Garrison Walls are Scilly's best-known and most impressive old defences; however, don't miss the more modest Harry's Walls across the harbour. We say “modest”, but if these 16th-century defences had actually been completed, they would have formed some of the most advanced military architecture in the country.
These ambitious plans were part of the project's downfall. The walls, complete with state-of-the-art Italian-style bastions, were abandoned before they were completed, partially due to costs. Harry's Walls also had the same problem as King Charles's Castle on Tresco: it simply wasn't in the best position to defend the islands.
Today, the remains of the walls are managed by English Heritage, who have installed helpful information boards at the site to help you get your bearings and work out what the original designs were. You can make out two bastions, connected by a 27-metre long curtain wall, which was one of the few sections to be built. Typically, a lot of the stone was re-used in later fortifications.
Dating to 1551, Harry's Walls are one of the earliest anti-French/Spanish defences on Scilly. Henry VIII has been busy fortifying the south coast for years, then the Scilly defences were begun during the reign of his son, Edward VI. Strictly speaking, these should be "Eddie's Walls"... The site is free to visit, and is open during daylight hours.