Elan-Rossdhu is a crannog, upon which Elan-Rossdhu Castle was built, probably some time before the 15th century.
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 the castle was built, and while little is known about it, the island is mentioned in the Charters of the Barony of Luss of 1541 and 1602. 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.
By 1541, and possibly as early as 1457, Rossdhu Castle had been built on the mainland, presumably as a replacement for the island castle.
The remains of Elan-Rossdhu Castle today show that it was a square building, measuring around 9.0m by 9.0m, although the walls now only stand to a maximum height of around 1.0m.