The hilltop is best known for it’s spectacular late 13th century castle. Long before this was built the site was occupied, probably as an Iron Age fortress, and possibly earlier. Roman coins and four skeletons have been found at the site indicating early occupation, but if there was an Iron Age hillfort at the site, all trace of it has been obliterated by subsequent building.
The sacred well or spring was probably sited at the end of the cave that was eventually incorporated into the castle itself. Water still collects at the end of the cave, but if this is the site of the spring, it’s no longer active. The atmosphere in the cave is certainly electric. When I lived in Carmarthen we sneaked up there at night and sat at the end of the cave in the dark. It was an amazing feeling being so deep down in the cold wet rock.
I’ve read in one source that the name Carreg Cennen has its roots in the Welsh for sacred well. I’ve not been able to confirm this though.