The monument comprises the fragmentary remains of a small tower house standing upon the summit of Island Muller, a small rock promontory which projects into the mouth of the Kilbrannon Sound. On the promontory there are also the remains of associated buildings. The monument was first scheduled in 1972. On this occasion, an inadequate area was included to protect all of the archaeological remains: the present rescheduling rectifies this.
Almost nothing is known of the history of this castle. However, during the 16th century the lands of Ballimenach and Smerby appear to have been held by the MacDonald family. Sir James MacDonald, son of Angus MacDonald of Dunnyveg, imprisoned his father at Smerby in 1597.
The tower, now reduced to its lowermost courses, appears to be constructed of local random-rubble masonry laid in lime mortar. It is oblong on plan and measures 13.3m from E to W by 12.0m transversely over walls 2.6m thick. An external return in the masonry of the W wall may mark the site of an entrance doorway, while a small relieving-arch near the centre of the S wall probably indicates the position of a latrine-chute outlet. The low turf-grown mound that partly encloses the tower may represent the remains of a rampart-wall of contemporary, or of earlier, date.
It is uncertain whether the causeway is of natural or of artificial origin. At the inner end of the causeway there is a small rectangular platform enclosed on its three landward sides by the remains of a wall of stone or turf, which may have served as a boat noost. The approach track passes the inner end of this platform and skirts the W base of the rock outcrop before turning eastwards to ascend its southern slopes. Immediately before the point at which it begins to ascend, the track passes through what seems to be the remains of a small sub-rectangular building or enclosure measuring about 10.7m from W to E by 7.6m transversely.
The area to be scheduled includes Island Muller, with its causeway to the mean high water level, the tower house and the associated buildings and structures. The area is irregular in shape and has maximum dimensions of about 150m NW-SE and about 87m NE-SW, as marked in red on the attached map.