Castle Down

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Castle Down is a windy, high plate of coastal land on the northern part of Tresco, Isles of Scilly, in Cornwall. It’s a protected area known as Castle Down (Tresco) Site of Special Scientific Interest (SSSI) for its biology and geology. The site covers about 58 hectares (0.58 square kilometres) and is entirely owned by the Duchy of Cornwall. It is also part of the Isles of Scilly Area of Outstanding Natural Beauty, the Isles of Scilly Heritage Coast, and is included in Plantlife’s Isles of Scilly Important Plant Area.

The landscape is a 35-metre-high plateau of coarse granite. The area shows signs of Ice Age activity, with glacial gravels and raised beaches along the coast. Thin, salty winds have shaped the vegetation into a distinctive waved maritime heath, where plants lean into the shelter of the wind and the ground is often bare on the windward side.

Castle Down has a long human history. Bronze Age people built many cairns, and there is an Iron Age field system nearby with hut circles and large middens. On the western high point sits King Charles’s Castle, built between 1548 and 1554 to guard the northern harbour approach to New Grimsby, but it was not well placed to defend against land or low shots from ships. It later became a quarry for Cromwell’s Castle. The round tower of Cromwell’s Castle was finished around 1652 to protect the harbour, and it was later updated around 1740 with a platform for cannons. In the mid-1600s there were tin mining pits nearby, though the work ended after a short time. Piper’s Hole is a dramatic, deep cave along the north coast with an underground pool; in the 19th century tourists rode a punt into the cave’s inner chamber.

Castle Down is also important for its plants and lichens. The wind-swept heath is dominated by heather (Calluna vulgaris) and, on the southern side, western gorse and bell heather. Other plants growing there include common bird’s-foot trefoil, English stonecrop, heath bedstraw, lousewort, and tormentil. About 45 lichen species have been found, including rare Heterodermia communities. A particularly notable lichen, Heterodermia propagulifera, is unique to the Isles of Scilly. The area also hosts a breeding colony of Common Terns.

The Castle Down SSSI was first notified in 1971 and was re-notified in 1986. It covers most of the higher land and the cliffs north of Tresco’s inhabited area.