This fort is absurdly (suspiciously) circular on the OS map. The notes in the RCAHMS record repeatedly complain about how densely vegetated it is so they couldn't survey it properly. But now the map makes the interior look cleared, so maybe explorers today can see more clearly.
The 'Thirlestane' of the name may come from the stone that was nearby (though of course everything could be confused with the properly castle-ish Thirlestane castle down the road).
Thirlestane. -- Standing on a slope 150 yards east of Thirlestane farm steading is a greenstone boulder 4 feet by 3 feet by 15 inches. This would seem to be identical with "Standandstayn" mentioned in a Confirmation by John Mautland of the lands of Snawdon about 1350.
Perhaps it's still there somewhere. From 'History of the Berwickshire Naturalists' Club' v 26 (1923).