When the British Islands were a Roman province, Carmarthen was the centre of the civitas of the Demetae Celtic tribe, known as Moridunum (sea fort). Carmarthen is possibly the oldest town in Wales and was recorded by Ptolemy in the Antonine Itinerary. The roman fort can still be seen and is believed to date to AD75-77.

The strategic importance of Carmarthen was such that the Norman William fitz Baldwin built a castle probably around 1094. The existing castle site is known to have been used since 1105. The castle was destroyed by Llywelyn the Great in 1215. In 1223 the castle was rebuilt and permission was received to wall the town (a murage). Carmarthen was probably the first mediaeval walled town in Wales. In 1405 the town was taken and the castle was sacked by Owain Glyndwr.

In the 16th and 17th centuries the dominant business of Carmarthen was still agriculture and related trades including woollen manufacture. In the mid 18th century the iron and coal trades became much more important although Carmarthen never developed Ironworks on the scale of Dowlais or Merthyr Tydfil.

Camarthen is the home of the famous Black book of Carmarthen written around 1250AD.

Famous Citizens
The architect John Nash.

Arthurian Legend
According to some variants of the Arthurian legend Merlin was born in a cave outside Carmarthen: "Caer" is Welsh for '"fortress" and "Myrddin" is Welsh for "Merlin." Historians generally diagree with this interpretation of the name but the story is popular. Many areas surrounding Carmarthen still allude to this, such as the nearby Brynn Myrddin (Merlin's Forest).

Legend also had it that when a particular tree called 'Merlin's Oak' fell it would be the downfall of the town as well. In order to stop this the tree was dug up when it died and pieces are now in the museum.

The Black book of Carmarthen includes poems with references to Myrddin (Ymddiddan Mrytin a Talyessin) and possibly to Arthur (Pa w^r yw'r Porthor ?). The interpretation of these is difficult because the Arthur legend was already known by this time, and many details of the modern form of the legend had been described by Geoffrey of Monmouth before the book was written. In addition some of the stories appear to have been moved into Wales at some point before their recording in the book.