When you pay to go into the castle, ask for a torch (small fee) - or bring your own torch. Once you enter the castle, look for the tower in the bottom left corner - this is where you will find the entrance to the cave. Initially there are steps down but this then changes to a concrete slope and then you are onto wet, natural stone - slippery. The cave ends with what appears to be a blocked up well?
Coflein says that there are at least nine caves in the rock. In about 1907 bones of two adults and a child, and a perforated horse tooth were found in the cave's stalagmite deposits. Three human teeth were found in 1980. The record says "the remains are conventionally dated to the Upper Palaeolithic, the period before the end of the last Ice Age."
In one part of the [castle] building a passage terminates in a flight of steps leading down to a dark subterranean cave of about 200, or perhaps, 250 feet long, and at the end of this passage or cave, there is a well which is still used as a "wishing well," more especially by young people.
[The author met three young ladies down there, but they were about to turn back being a bit weedy, but he gallantly accompanied them on to the well:] Before we left the spot, each one of the three young ladies threw a bent pin into the well, wishing, I suppose that she might have her heart's desire. We found many pins at the bottom of the well, which had been probably left there by young people given to the practice of amorous spells.
From 'Folklore of West and Mid-Wales' by J C Davies (1911).
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 Water still collects at the end of the cave, but if this is the site of the spring, it's no longer active.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.
This Web page is largely about the castle, but it includes photos of the cave, and the passage leading to it. Don't be alarmed if you find that some images on this page are obscured by text. The page isn't very well glued together!