Courtesy of the nice people at RCAHMS:
Occupying the summit of Tom a’ Chaisteil, a rocky spur, is a fort, sub-circular on plan, measuring c. 33.0m NNE-SSW by c. 31.0m, within a tumbled wall, most of which in the E has fallen over the cliffs of the spur. Occasional outer facing stones can be seen but no inner face, so that the thickness of the wall cannot be determined, although the width of tumble suggests that it may have been about 3.0m. The entrance was probably in the W where a gap has been blocked by a later wall, but no details survive. The interior is featureless. Some eight to ten metres to the N of the fort is a trench c. 2.0m wide and c. 0.6m deep, extending 35.0m W from the cliff. It is presumably associated with the fort, the most likely explanation being that it is part of an unfinished outer defence. On the W side, below the wall, is a recent shelter
I’d take issue with the comment about ‘featureless’ interior. I’m sure there were bits that looked like hut circles.