Achadun Castle is a late 13th century stone square enclosure fortress, founded by the MacDougalls. On the western shore of Lismore, the castle sits conspicuous on a limestone ridge, overlooking Bernera Bay. From the 15th century, Achadun was held by the Bishops of Argyll until its abandonment in the 16th century, following Bishop David Hamilton construction of Saddell Castle. Little is known of its early history and the south and the east enclosure walls, plus most of the internal walls have collapsed. Internally there was a substantial south-eastern first floor hall block, a small courtyard and other two storey building ranges. In the north enclosure wall, are two first floor window embrasures and the landward gateway, with a straight mural stairway giving access to the parapet walk. A western gateway is defended by a stone-built platform, which incorporate a deep square pit designed to contain a movable platform or bridge. Excavations in 1970-1, found a sub-rectangular dry stone masonry building of a late date, occupying the north-east part of the courtyard.