In 1400, three courtiers of King Robert II took advantage of their lord’s absence to Rothesay Castle. Crossing into Cowal on a hunting trip, they encountered and raped three Lamont women. In a rage, Lamont clansmen caught up with the three courtiers and brutally murdered them. The incident was passed along to the King, who punished Clan Lamont by rescinding nearly eight square miles of their lands in Strath Echaig and granting them to the Campbells.

As a result of this increasing lordship of Lamont lands, Clan Campbell became even more bold in asserting their power over Argyll, and more specifically, Cowal. Whether by force or through sheer kindness, in 1442 the chief of Clan Lamont gave permission for the eldest son of Sir Duncan Campbell to be buried at the ancient Kirk of Kilmun on the Holy Loch, which was within the confines of Lamont territory. The Highlands were impassable because of snowfall, and those conditions led to the request. After this, Campbell petitioned the Pope to found a Collegiate Church on the site. The Pope conceded, and Sir Duncan Campbell endowed the site. He turned the Kirk into a burial place for Campbell chiefs, and it remains so even to the current day.

In 1472, Clan Campbell received charter for the lands around Dunoon, and they proceeded to turn the castle into their main seat