Rossdhu Castle is thought to have been built as a replacement for Elan-Rossdhu Castle, which stands on a crannog just off the shoreline.
The crannog was granted to Maldouen, the First of Luss, by Alwyn, the Second Earl of Lennox, some time before 1220. It was later confirmed as a possession of Gillemore, the Second of Luss, in a charter by Maldouen, the Third Earl of Lennox, in or just after 1225. Later still the crannog was confirmed to belong to Gilliemore’s son Maldoven.
Probably around this time a castle was built on the crannog, about which little is known. In the 14th century the daughter of Sir Godfrey, the 6th of Luss, known as The Lady of Luss, married Sir Robert, the 5th of Colquhoun, and Elan-Rossdhu Castle became a Colquhoun property.
Rossdhu Castle, built on the mainland by the Colquhouns, is first mentioned in Charters of the Barony of Luss in 1541, but may have been built as early as 1457. It seems that stone from Elan-Rossdhu Castle was used to build the new castle, which is thought to have taken the form of a strong square tower.
Rossdhu Castle was abandoned in 1770 by the Colquhoun family, who had built Rossdhu House nearby as a replacement. Stone from Rossdhu Castle was used in the construction of the new house, leaving the old castle in ruins.
Today the ruins of Rossdhu Castle stand at the edge of Loch Lomond Golf Club, and consist of the south wall, measuring some 10.0m in length by 2.3m thick, and rising to a height of around 12.0m, with smaller adjoining sections of the east and west walls measuring around 3.0 metres and 2.0 metres long respectively.