It's a really odd shape for a hillfort. Like one standard hillfort that's ben cracked in half, then each half set against the edge of the scarp, one next to the other. Or perhaps one hillfort with a hinge, which has been swung open, to make two hemispherical forts. It's been described as being like a pair of half-moon spectacles when viewed from above.
It's got fair sized banks and ditches, but it's most salient feature is the great big rock cut basin just on the edge of the scarp, see Pebble's photo. Given the nearby astounding rock art, you can't help but wonder if there's a connection between this thing and the cupmarks which litter the area.