Carmarthen, United Kingdom

During the Middle Ages, the settlement then known as Llanteulyddog ('St Teulyddog's) accounted one of the seven principal sees (Cantrefi) in Dyfed. The strategic importance of Carmarthen caused the Norman William fitz Baldwin to build a castle there, probably about 1094. The current castle site is known to have been occupied since 1105. The castle itself was destroyed by Llywelyn the Great in 1215, but rebuilt in 1223, when permission was given for a town wall and crenellations, making it one of the first medieval walled towns in Wales. In 1405, the town was captured and the castle sacked by Owain Glyndŵr. The Black Book of Carmarthen of about 1250 is associated with the town's Priory of SS John the Evangelist and Teulyddog.

The Black Death of 1347–1349 arrived in Carmarthen with the thriving river trade. It destroyed and devastated villages such as Llanllwch. Local historians cite the plague pit for the mass burial of the dead in the graveyard that adjoins the Maes-yr-Ysgol and Llys Model housing at the rear of St Catherine Street.

In 1110, the ancient Clas church of Llandeulyddog, an independent, pre-Norman religious community, became the Benedictine Priory of St Peter, only to be replaced 15 years later by the Augustinian Priory of St John the Evangelist and St Teulyddog. This stood near the river, at what is now Priory Street (51.8601°N 4.2975°W / 51.8601; -4.2975 (St John's Priory), SN418204). The site is now a scheduled monument.

Franciscan Friars (Grey Friars, or Friars minor) became established in the town in the 13th century, and by 1284 had their own Friary buildings in Lammas Street (51.855794°N 4.309076°W / 51.855794; -4.309076 (Carmarthen Greyfriars)), on a site now holding a shopping centre. The Franciscan emphasis on poverty and simplicity meant the church was smaller (some "70 to 80 feet long and 30 feet broad" – 21/24 by 9 m) and more austere than the older foundations, but this did not prevent an accumulation of treasures, as it became a sought-after location for burial. In 1456 Edmund Tudor, 1st Earl of Richmond died of plague in Carmarthen, three months before the birth of his son, the future King Henry VII. Edmund was buried in a prominent tomb in the centre of the choir of the Grey Friars Church. Other notables buried there were Rhys ap Thomas and Tudur Aled.

The Friary was dissolved in 1538, and many unsuccessful plans were made for the building. Even before the friars had left in 1536, William Barlow campaigned to have the cathedral moved into it from St David's, where the tomb and remains of Edmund Tudor were moved after the Carmarthen buildings were deconsecrated. There were repeated attempts to turn the buildings into a grammar school. Gradually they became ruined, although the church walls were still recognisable in the mid-18th century. By 1900 all the stonework had been stripped off and there were no traces above ground. The site remained undeveloped until the 1980s and 1990s, after extensive archaeological excavations of first the monastic buildings and then the nave and chancel of the church. These confirmed that the former presence of a church, a chapter house and a large cloister, with a smaller cloister and infirmary added later. Over 200 graves were found in the churchyard and 60 around the friars' choir.

Geoffrey of Monmouth, writing in 1188, began the legend that Merlin was born in a cave outside Carmarthen. The town's Welsh name, Caerfyrddin, is widely claimed to mean "Merlin's fort", but a reverse etymology is also suggested: the name Merlin may have originated from the town's name in the anglicised form of Myrddin. (See Merlin § Name). An alternative explanation is that Myrddin is a corruption of the town's Roman name, Moridunum, meaning "sea fort."

Legend also had it that if a certain tree called Merlin's Oak fell, it would bring the downfall of the town. Translated from Welsh, it reads: "When Merlin's Oak comes tumbling down/Down shall fall Carmarthen Town." To obstruct this, the tree was dug up when it died; pieces of it remain in the town museum.

The Black Book of Carmarthen includes poems that refer to Myrddin (Ymddiddan Myrddin a Thaliesin, "Conversation of Merlin and Taliesin") and possibly to Arthur (Pa ŵr yw'r Porthor?, "What man is the porter?"). Interpretation of these is difficult, as the Arthurian legends were known by this time and details of the modern form had been described by Geoffrey of Monmouth before the book was written. Some historians suggest that Vortigern along with his army from Powys may have invaded the Ystrad Tywi in order to gain control of it but had to retreat either due to local rebels fighting back or being defeated by Dyfed, but in the process may have kidnapped a young Merlin from Carmarthen hence why the character is legendary within the town.

One of the earliest recorded Eisteddfodau took place at Carmarthen in about 1451, presided over by Gruffudd ap Nicolas.