Apparently this means something like 'the lookout' from its most recent use following a foray by hostile clansmen from another island who chopped off the thumbs of all the men so they could never again use a sword.

This view along its west face shows the outlook over the nearest 'ports', but the view is far better from the summit of the hill of which this is a southerly spur. Maybe some of the interior spaces were used as a shelter for the alternative watch. See also http://www.pbase.com/crisscross/image/155783454