This is on 'my patch' and I have visited it numerous times over the past few years. It is very accessible and is mentioned on the nearby info/interpretation board. Until recently there has not been too much visible evidence of its existence other than the piles of cairn material which have obviously been excavated/robbed out of its centre leaving a deep hollow. I visited again on Sunday 30 October 2011 and was pleasantly surprised.
It is listed by Coflein (NPRN 307082) as having "...possible original kerbing visible within the cairn" and having been "....centrally robbed c1884."
There has now been some clearance of bracken/shrubs from the site and this has revealed what look to me like the kerbstones referred to by the Coflein record. There are 7/8 fairly large kerbstones forming an arc (of an oval, not circle in my view) within the cairn and I can see/feel evidence of others beneath the soil/heather possibly aligned with the visible stones. This would need a more experienced eye than mine to interpret and look forward to reading other's views.
The platform on which the cairn was built has also been cleared to several feet outside what is assumed to be the original cairn base and this whole area is clearly visible from the summit of the small but neat Nercwys Mtn close by. The area is strewn with cairn material and I have read somewhere that the damage was originally done by the shepherd living in the Shepherds Cottage (also currently the subject of a recent archaelogical dig) on the mountain in the 1800s searching for a chest of gold.
Beyond the small valley beneath Nercwys Mtn are the distant mountains of Eglwyseg and the Berwyns which may be part of the reason for the choice of site.
How to get there: the site is on the sloping south side of the mountain within a few hundred metres of gate/stile onto the mountain. It is best approached from the north carpark (space for approx 10 vehicles) and then taking the waymarked circular route clockwise around the mountain. You will see the cairn about half way around as you leave the forest set up high above you on the right. At this point you also have the summit of the small mount in front of you.