Kisimul Castle, on a rock barely larger than the castle itself, is the ancestral seat of Clan MacNeil and one of the most photogenic small castles in Scotland. The earliest stonework dates from the 15th century, though the MacNeils claim much older occupation. The castle was abandoned for two centuries, fell into ruin, and was painstakingly restored by an American MacNeil descendant — Robert Lister MacNeil, an architect — in the mid-20th century.
A short boat trip from Castlebay pier (operated by Historic Scotland) brings you to the islet. The tour takes about 40 minutes and includes the great hall, the chapel, and the watchtower. Open April to September, weather permitting; the tiny boat does not run in rough seas.