Castle Sween in Argyll is one of the oldest castles in Scotland. It was built in the 11th or 12th century, and has a curtain wall, enclosing a rectangular courtyard, and a keep. It is said to be named after Sueno, an 11th century Dane. He may have been the builder of the castle. It was held by the McSweens until it was captured by Robert the Bruce in 1315. It was then held by the McMillans, the Stewarts of Menteigh, the MacNeils of Gigha for the MacDonald Lord of the Isles, and in 1481 the Campbells became keepers for the Crown.

During the time of the McMillans the western wall was dismantled and a round tower and a new rectangular building were added at the northwest corner. The tower is standing today, and is known as McMillan's Tower. The Campbells became keepers for the Crown in 1481. In 1647 it was captured and partially destroyed by Alaisdair Colkitto MacDonald. One tower collapsed in the 19th century. It has been repaired and is open to the public.