An Sean Chaisteal (the old castle) is an iron age broch, now in a ruinous state with only a few courses of stones visible.

The broch can be accessed via a short walk from Ardnacross farmhouse, where with consideration you can park. The ruinous broch overlooks the Sound of Mull, on the brink of a rocky cliff some 20 feet high, bordering the shoreline.

Approaching from the farm along a track takes you across flat boggy ground. The broch walls have been so severely reduced that the tumbled debris now appears as a circular mound of stones, with the centre slightly hollowed.  However, a sufficient number of facing-stones are exposed in the surface of the mound to establish the layout of the broch and the thickness of its walls.

The entrance, on the north-west, is choked with debris and partially hidden by ferns. Fallen lintel-stones mark the entrance with recesses in the stonework still in place which would have held a wooden bar across the door.

There are also slight traces of intramural broch features, but these are difficult to determine due to the fallen debris and undergrowth.

To the north of the broch is a thin scatter of stony debris in what may have been a defensive wall to provide extra protection to the broch entrance.

The views from the broch along the Sound of Mull are really impressive and it is easy to understand why a defensive dwelling would be built here. A fascinating site well worth making the short walk from the farm.