Yr Hen Dderwen is named after Merlin's Old Oak that used to grow in Priory Street.
Carmarthen is said to be the birthplace of Merlin (Myrddin). The prophecy states that when Merlin's Oak shall tumble down, then shall fall Carmarthen Town.
In the early 19th Century, a local man appears to have poisoned the tree. The last fragment of the tree was removed in 1978 and Carmarthen is still standing strong.