Visited 6th April 2003: Although it's an impressive place, this isn't the kind of site I would usually go out of my way to visit. Luckily it's sat betwixt Maengwyngweddw and Maen Serth, which makes for a lovely ridgeway walk.
There are two cairns that I could identify. One is really quite large, and is either a ring cairn or an excavated cairn (I'd go with the latter). It has some sizeable kerb stones on its western side, but nothing much elsewhere. Just to the south of the big cairn is a tiny one.
In his book The Spirit Paths of Wales (ISBN: 1-85284-289-X) Laurence Main states that, "two cairns stood here until 1910, when Council workmen destroyed the larger one and carted away its stones for road metal". He also describes a Bronze Age axe-hammer found at the site, made from "spotted blue dolerite from Preseli".
As ever, Main doesn't include any of his sources, but I've read about the Council worker story somewhere else. Strangely, I discerned two cairns on my visit, so I'm not sure whether the smaller of the two was the 'destroyed' cairn of whether there were originally three cairns (probably the former).