The monument comprises the remains of Achadun Castle, the 13th century castle of the bishops of Argyll. The castle was originally scheduled in 1964, but the scheduling did not cover all of the archaeological remains; the present rescheduling rectifies this.
In 1240 two pennylands at 'Achacendune' were among lands in Lismore granted by Ewen, son of Duncan of Argyll, to Bishop William; while in 1304, his grandson granted to Bishop Andrew other property near the bishop's 'castle or manor of Achychendone'. Almost all of the remains may be ascribed to some time between these two dates. The castle remained in use until the early 16th century, when the construction by Bishop David Hamilton of his castle at Saddell, about 1508-12, effectively terminated the occupation of Achadun as an episcopal residence.
The castle occupies the summit of a prominent limestone ridge near the western shore of Lismore, close to a good anchorage in Achadun Bay. The structure is ruinous, the SW and SE walls having collapsed outwards. The NE wall and a substantial portion of the NW wall, however, survive almost to wall-head level.
A curtain wall varying in thickness from 1.4m to 2.4m encloses an area measuring about 22m square, which contained at least two ranges of buildings flanking a small courtyard. The SE range measured 7.6m wide internally and comprised a ground floor and an upper floor. The NW wall of this range is 1.6m in thickness at ground level. Although largely buried under rubble, it still stands to a maximum height of 2m. Excavations in 1970-71 revealed two doorways leading into this range from the courtyard. The excavated finds confirm recurrent occupation of the site through the late medieval period.
A smaller range occupied the N angle of the courtyard but appears to have been a less substantial structure and no traces of inner walls survive.
The landward entrance is centrally placed in the NE curtain wall. From this entrance, a mural stairway gave access to a parapet walk. A second entrance gateway is located in the SW. The approach to this entrance from outside was via a stone-built platform which, on excavation, was found to incorporate a deep pit 2.2m square, and was designed to contain a movable platform or bridge.
The remains of a sub-rectangular building in the NE part of the courtyard, and partially overlying the NW range, were excavated in 1971 and found to be a later addition.
The area to be scheduled has maximum dimensions of 34m NW to SE by 32m transversely, to include the castle and a 6m strip of land around it within which related remains may be expected to survive, as marked in red on the accompanying map.