Once of great strategic importance as it controlled access to the inner western seaway, Cairnburgh Castle lies at the mouth of Loch Tuath in W Mull, 3 miles (5 km) southwest of Treshnish Point and 12½ miles (20 km) north of Iona. Unusually its defences are split between two small islands which lie only a few metres apart at the northern termination of the Treshnish Isles; namely Cairn na Burgh More and Cairn na Burgh Beg. On the former the castle comprises a courtyard, barrack block, chapel and guard-house, while a lesser defence, including another guard-house and a well, was constructed on the smaller island. Most likely originally a Viking fortress, called Kiarnaborg, it first appears on records in 1249 when it was held by the MacDougalls of Lorne. Forfeited to the Crown in 1309, the castle was occupied by the MacDonalds of the Isles on behalf of the King, but it later passed to the MacLeans of Duart. The MacLeans rebelled in 1504 and Cairnsburgh was besieged by King James IV (1473 - 1513). It was captured by David Leslie (1601-82) in 1647 and then taken by Oliver Cromwell's army in the 1650s, when books and manuscripts from Iona which had been brought here after the Reformation for safe-keeping were destroyed. It was attacked by the Campbells in 1679, but held out, although was surrendered once again in 1692. Cairnsburgh was garrisoned by Government troops during the Jacobite Rebellions of both 1715 and 1745. Although the chapel may date from the 15th century, the other surviving structures are from the 16th or 17th centuries. The castle was subject to an archaeological investigation by the University of Glasgow in 2006, on behalf of its owners, the Hebridean Trust.