Carmarthen, the county town of Carmarthenshire, claims to be the oldest town in Wales. Its roots go back to the Roman Occupation when Carmarthen was the civitas capital of the Celtic Demetae tribe and known as Moridunum ('Sea Fort'). The Roman fort in the town dates from about 75 AD and was recorded by Ptolemy and the Antonine Itinerary. Near the fort and excavated in 1968 is one of only four surviving Roman amphitheatres found in Wales. During the Middle Ages, the settlement was known as Llanteulyddog ('St. Teulyddog's'). There are strong Arthurian legends connecting Merlin with Carmarthen, but it was the town's strategic importance that was recognised by the Norman William Fitz Baldwin who built a castle there about 1094. The castle was destroyed by Llywelyn the Great in 1215 and rebuilt in 1223, at the same time as the town walls were constructed making Carmarthen one of the first walled towns in Wales.

Neither of these defences prevented Owain Glyndwr from sacking both town and castle in 1405. In the 16th and 17th centuries, Carmarthen rose to prominence as a business centre for agriculture and associated trades, including woollen manufacture. The 18th century saw Carmarthen enter the industrialised age. Coal, iron and tinplate trades became important and, as a port, Carmarthen exported its products throughout the World. Today, the former cattle market in the heart of the town has undergone regeneration and Carmarthen has become a large business and shopping centre for the county.