On the isle of Inishmore, a footpath cuts through the hill up to the edge of the cliffs (61m), where there is a great drystone fort. It is one of Europe's finest prehistoric monuments, with three lines of defence. Stone stakes are planted at an angle between the outer wall and the middle wall in order to impede any attack. A square tunnel in the thickness of the wall leads into the inner compound. The inner wall forms a semicircle which begins and ends on the edge of the cliff.