I can't believe there aren't more posts for this. I went to see this last year, it's an absolutely huge hill fort and an amazing site. It's unusual because all the ramparts and fortifications are still there and stone built, rather like Grimspound in Dartmoor on a massive scale. The huge cairn on the other side is called the red cairn I think and is a huge pile of stones that could be mistaken for a quarry.
Drive through the village of Bethleham, up the hill, over the cattle grid and there is a parking place on the left. It is then an easy walk up the hillfort. Be careful though as the collapsed stone walls are very loose underfoot. The countryside here is beautiful. When I visited there wasn't a cloud in the sky and red kites were gliding silently overhead - bliss.
There's conflicting opinion about what this site is. The RCAMW list it as a Pillow Mound (something they seem to categorise as a natural feature). Cadw list the site as a Long barrow, and Cambria Archaeology list it as a possible Chambered Tomb or Long Barrow.
Carn Goch (red cairn) is one of the largest stone-built hillforts in south Wales, and the finest example of one with stone ramparts, which are a little worse for wear now. There are two forts Y Gaer Fach (the small fort) and Y Gaer Fawr (the large fort!). The views over this remote part of the Brecon Beacons sound fantastic.