The remains take the form of two defensive ditches protecting a clearly visible rectangular rampart.

The rampart behind the ditches still stands up to five feet high in places, though obviously, this would have been far higher when the fortlet was in use.

On the rampart stood a wooden palisade, at least ten feet high, with a walkway running the length of the fortlet.

There is an entrance through the rampart at the eastern end, over which would have stood a wooden tower attached to the walkway.

Castle Greg was a most likely used as a monitoring base for an east–west road running along the foot of the nearby Pentlands, from the Forth to the Clyde Valley.