Visited 16.6.2010 on a day-trip to Tresco on the Scillonian ferry. The boat across from St Marys dropped us off at Carn Near, the southern point of the island, which put us as far from Castle Down as it was possible to be.
It was a lovely walk under blue skies along the eastern edge of the island, and the views across to St Helen's, Round Island and Northwethel from Gimble Porth are beautiful. We climbed up to the Down from its eastern side (Tregarthen Hill) and a prehistoric landscape opens out. Unfortunately it's very heather-covered, making it difficult to see what's what. There are 78 cairns on the downs, but most are simply lumps in the heather. A nice prehistoric field boundary does break free of the vegetation and as we head northwest across the Down we do manage to find a decent cairn with surrounding kerb and then what looks like a small capstone protruding from another be-heathered cairn.
Unfortuately our time is rather limited and in going up to have a look around the 16th century King Charles's castle (which predates the monarch of the same name by the best part of a century), we don't have time to seek out the entrance grave up on the west side of Tregarthen Hill. Still, it's a great spot on a nice day and we'll have to come for longer another time.
As we head back towards New Grimsby, we pass another prominent cairn towards the southern end of the Down. New Grimsby has a rather lovely modern megalithic folly, with a kerbed quoit (mini-Chun) and several standing stones, overlooking the harbour. From there we headed off to Tresco Abbey Gardens, with its holed stone.
It seems that the majority of visitors to the island come to see the Gardens and the little town, so Castle Downs are a nice quiet spot even on a lovely sunny summer's day.
The cairns are well preserved but overgrown with gorse so getting in is nigh-on impossible. There are some signs of field clearance- a pile of stones 200m N/NE from the castle and a field wall that can be seen running parallel to the path in one of my images.
The Piper's Hole is at the north east of these cairns.
Under the cliffs of Peninnis Head on St Mary's there is a cavern, termed the Piper's Hole, which extends a long distance under ground, and is absurdly said to communicate with another cave of the same title, the entrance to which is in the island of Tresco. This legend would make the length of the cavern at least four miles; and the inhabitants of the locality tell you of dogs let in at the one entrance coming out, after a time, at the other with most of their hair off, so narrow are some parts of the cave. So there is a tradition in Scotland of a man getting through a similar cave, but paying the penalty in the loss of all his skin.
From 'Rambles in Western Cornwall' by J O Halliwell-Phillipps (1861).