This is an enigmatic place, described as a possible round barrow (Cadw & Cambria Archaeology), chambered tomb (Cambria Archaeology), Roman watch tower (RCAHMW & Cambria Archaeology), enclosure (RCAHMW) and standing stone (Cambria Archaeology).
Carreg-y-Bwci consists of a mound with a large hollow in the centre of it (similar to a donut on its side). In the middle of this is a large prostrate stone. Not an easy site to interpret, but certainly something special. It is sited on a ridge at the head of the river Ffrwyd Cynon.
The ‘Roman watch tower’ theory has gained some credence because Carreg-y-Bwci sits on an old Roman road (Sarn Helen). This doesn’t seem very conclusive to me, as the Romans had a habit of intentionally running their routes past existing megalithic sites (or re-using existing trackways). Also, Carreg-y-Bwci isn’t at the highest or even second highest point in the area. I know very little about Roman watch towers, but I would have thought a vantage point without the obstruction of nearby hills would be sensible.
In it’s modern form the Roman road marks the boundary between Ceredigion and Carmarthenshire. There’s a possible stone circle 1km to the north west in Coed LLwynifan (SN63754838).