The monument comprises Ardtornish Castle and associated buildings, dating from about the 13th century to about the 17th century. The monument was first scheduled in 1970, but the area then protected did not include the visible associated remains. The present rescheduling rectifies this.
The monument lies at the seaward end of a promontory which projects southwards into the sound of Mull, about 2km SE of the mouth of Loch Aline. It consists of a hall-house measuring approximately 17m WNW-ESE by about 9m transversely, defined by walls measuring some 2.7m thick and standing to a height of about 8m on the S, 5m on the N and W, and some 2m on the E. The hall-house was partially repaired between 1910 and 1915, when large amounts of facework were renewed, including most of the E wall, the NE and SE angles, and the upper portion of the S wall. The entrance to the hall is near the N end of the E wall, and there is an arched window in the S wall.
To the N, E and SE of the hall-house are the remains of at least 14 rectangular and sub-rectangular structures, surviving as low, grassed foundations. The largest of these, some 25m to the N of the hall-house, measures approximately 22m N-S by about 11m transversely and has two 'wings' at the NW and SE. The other structures lie in the immediate vicinity of the hall-house and beneath the level of the rock promontory, along the E and SE foreshores.
Ardtornish Castle was a major seat of the Lords of the Isles as shown by the survival of six Acts of the Lords of the Isles which were signed here. Ardtornish also holds a special place in the downfall of the Lords of the Isles since it was here in 1462 that John, 4th Lord of the Isles, signed his part of the Treaty of Westminster-Ardtornish, a pact with Edward IV of England, the discovery of which led to the forfeiture of the Lordship in 1475.
The area proposed for scheduling comprises the remains described and an area around them within which related material may be expected to be found. It is irregular being defined to the SW and SE by the high water mark, to the NW by a line 50m out from NW wall of the hall house and to the NE by a line 100m out from its NE wall. The area measures about 230m from its northernmost point to its southernmost point and about 230m from its easternmost point to its westernmost point, is marked in red on the accompanying map extract.