On the east side of Loch Sween, this castle guards the mouth of the Loch, looking past nearby islands and down the Sound of Jura towards Ireland. It is one of the earliest stone castles in western Scotland, having been built about 1100 or so.
Its plan is that of a quadrangle, with projecting buttresses, an unusual feature in a western highland fortification. The gateway is on the south side, in a ten foot thick section of masonry. There is a well in the north east angle. MacMillan's Tower, of later, 15th century date, is outside the north east angle. The round tower at the north west angle contained a prison at one time.
Its situation would have made it convenient to beach galleys, there being no natural anchorage nearby. The early history and siting of this castle have much to do with the emphasis upon sea power which dominated western Argyll.
A leader named "Suibhne", grandson of Hugh the Splendid (died 1047 AD), is credited with building Castle Sween. By the 13th Century, the Clan MacSween lands extended as far as Lochawe in the north and Skipness in the South. The family was, however, on the wrong side of the Wars of Independence. Robert the Bruce displaced the MacSweens, and granted the Castle to his loyal supporter, Angus (Macdonald) of Islay. From here, the Castle ended up in the hands of the Campbells (1490). Throughout, the Constables of Castle Sween seem to have continued to be the MacNeills. The last MacNeill to hold this position was Hector Torquil MacNeill, 1st of Taynish (who died after 1449).
Hector MacNeill's only lawful descendent was his daughter, Erca. When she married Alexander MacMillan, her dowry consisted 10 farms extending from the Castle to the head of Loch Caolisport to the east. She and her husband lived at the Castle, and it was for them that 'MacMillan's Tower' was built. The Constableship of the Castle thus passed to the MacMillans.
During the British Civil War, the castle was attacked and burned in 1644 by Alisdair MacColla and his Clan Donald, during his ravaging of Argyll. The Castle has been a ruin ever since.