Somerled, the 12th-century Norse-Gaelic warrior who carved out a domain across the western seaboard, maintained a fortress at Ardtornish because of its commanding position on the strategic sea lanes connecting the Hebrides with mainland Scotland. The promontory extends south into the Sound of Mull about a mile southeast of the village of Lochaline, offering clear sight lines in every direction. For the high chiefs of Clan Donald who succeeded Somerled, Ardtornish became one of their principal seats from the early 14th to the late 15th century. It was here that John of Islay, Lord of the Isles and 6th chief of Clan Donald, died in the 1380s. His funeral procession sailed from Ardtornish through the Sound of Mull to the sacred island of Iona -- a final voyage befitting a ruler whose domain was defined by water.

Ardtornish was more than a military stronghold; it was an administrative centre from which the Lords of the Isles governed their realm. Donald of Islay, John's son and successor, issued charters from the castle in both Latin and Gaelic -- at least two of which have survived. According to tradition, it was from Ardtornish that Donald's galley fleet sailed to transport the vassals of the Isles to the west coast of Ross-shire. There they landed and launched the invasion in support of Donald's claim to the Earldom of Ross, a campaign that culminated in the indecisive Battle of Harlaw in 1411. The castle's role as a gathering point for naval expeditions underscores how profoundly the Lordship of the Isles was a maritime power, its authority exercised not through roads but through sea lanes, galley fleets, and island fortresses.

The castle's most consequential moment came in 1461. John of Islay, the fourth and last independent Lord of the Isles, met commissioners of Edward IV of England at Ardtornish to negotiate a secret treaty. Under its terms, John, his kinsman Donald Balloch of Dunnyvaig, and the exiled Earl of Douglas would each receive a third of the kingdom of Scotland in exchange for becoming loyal English subjects. The English king sweetened the deal with generous cash payments until the conquest was complete. It was an astonishing gamble, and it failed spectacularly. When the English government revealed the treaty to Scotland in 1474, John was stripped of the Earldom of Ross. A decade later, after further rebellions, the Lordship of the Isles itself was forfeited. Ardtornish passed eventually to Clan MacLean of Duart, then to the Campbell Earls of Argyll, and by the end of the 17th century the castle was abandoned. Today it stands as a scheduled monument, its walls crumbling on the headland where Scotland was once nearly bargained away.