Hi Hob - yes, Megadread and I had a pleasant evening pondering around Carl Wark. I think I came away with more questions than I arrived with. It's quite an impenetrable place. In comparison to Mam Tor and Gardom's Edge, both have quite easily recognisable features that help to date them.
The only visible feature that could offer any kind of clue by way of dating at Carl Wark is the wall. An excavation could probably find enough material to radio carbon date in the earthen structure supporting the stonework. Stuart Piggott thought that the structure bore a resemblance to similar Scottish Dark Age constructions. But then again, taken out of cultural context, does that necessarily suggest a similar date? If a large, ruined circular structure was found in the Peaks, it wouldn't make it a broch. Although there was conflict in the area during the Dark Ages, with a well know battle at nearby Win Hill c. 626AD.
In answer to your comment on my Carl Wark image. Yes, there are other Tors nearby, but no rock art. The Peak District in general doesn't have a great deal of rock art in comparison to somewhere like Ilkley Moor.
The topography of Hathersage Moor is interesting. Higger Tor towers over Carl Wark to the north and Over Owler Tor is also visible nearby to the south west. Both have odd looking rock forms, Over Owler in particular. Mother Cap rock, sitting on the back of Over Owler is also visible (as it is for miles around). Burbage Rocks are a gritstone edge that runs north to south, to the east of Carl Wark, meaning that Carl Wark rises like a mini volcano (especially when viewed from the east) in the natural amphitheater of Hathersage Moor.
There are other prehistoric sites on the moor. There are cairns and a standing stone at Winyard's Nick (a ridge that runs from Over Owler towards Higger Tor, with the moorland sloping to face Carl Wark). Also, cairns and a possible roundhouse at Toad's Mouth to the south. There is also a fair bit of prehistoric activity in the surrounding hills at Froggatt Edge, Wet Withens and Minninglow. I think there are also a number of find sites within the area ranging from Mesolithic to Bronze Age.
Once you step inside Carl Wark, it quickly becomes apparent that it was certainly not suitable for any kind of occupation. Gatherings of any size would probably be quite tricky too. The only reasonably safe assumption is that it's not a hillfort. Or at least not like any other!
