This is a place I have never been to, and it looks like no one else has either. Its another Chevaux de Frise, though not the same as the 'Bryn Cader Faner' cairn which is so striking.
According to N.P.Figgis (Prehistoric Preseli) the method employed in building these treacherous out pointing stones might have been the result of an itinerant architect, be that as it may Castell Henllys, which is close to Carn Alw, has similar defence methods in its ditch, now covered by a later bank.
Carn Alw though situated at the back of Carn Meini if my map reading skills are right, is defended by "a wide chevaux de frise, incorporating set and naturally grounded stones in roughly three bands" on the easily accessible south-east side."
Figgis goes on to speculate as to whether the elaborate entrance and boulder lined lane with a kink in its length has something to do with a stock enclosure, either for bulls or ponies, as the area defended is very small. Wild ponies still graze these moors, and they are very like the small ponies depicted on Iron Age coins......