Carmarthen is in South Wales. There are three sites with Merlin connections in the Carmarthen area. Bryn Myrddin (Merlin's Hill), Merlin's Stone, and Merlin's Tree (The Priory Oak).
The Bryn Myrddin is a tree-covered hill that sits right on the A40. On Bryn Myrddin, there is supposed to be a legendary hidden cave where Merlin was trapped by Vivien. As the legend goes, Merlin was in love with the sorceress and taught her spells. After earning his trust, she used the same spells he taught her to imprison him in, perhaps, the cavern in Bryn Myrddin. This is one of the many "cave-legends" in the Arthurian tradition.
Merlin's Stone. Merlin is supposed to have prophesied about this stone, saying that a raven would one day drink a man's blood off of it. Merlin supposedly kept his treasure hidden near here, and, indeed, the stone did fall on a treasure hunter once, crushing and killing him. This, in essence, fulfilled the prophesy, and it took five horses to stand the stone back up as it was
Merlin's Tree. There is a popular jingle that goes along with Merlin's Tree:
"When Myrddin's tree shall tumble down, Then shall fall Carmarthen Town."
To keep this prophesy from being fulfilled, the town of Carmarthen kept bracing
the dead tree when it sat in the center of town. They even went so far as to
pour cement around the base. Finally, the tree became too unstable to support
any longer and one branch was saved. This branch is now in the Carmarthen Museum
in a small display case. In the display case now is a small piece of the tree,
a picture of its old home in town, and a marker that tells a brief history and
the associated legends.