The town existed well before the Normans, the Romans building a fort in AD 75-77 and a settlement developed. After the Romans departed it became a centre of the welsh community-there is evidence of a Celtic church dedicated to St.Teulyddog. The Normans under command of William fitz Baldwin arrived in 1094, although they decided against building within the old Roman fort but built a new Castle which overlooked the river. Originally known as Rhyd-y-Gors, the castle was destroyed and re-built several times. A community developed outside the Castle gates and received its first grant of privileges from Henry 1 in 1109. The original welsh settlement to the east was allowed to continue and fell under the control of St.John's priory by royal grant of Henry 11.
Therefore during the medieval period there were two Carmarthen's-an Anglo-Saxon borough below the castle and a native township around the old church of St Teulyddogs. In 1223 Carmarthen received a murage, the earliest known in Wales, and the area of the modern Guildhall, Quay St and Bridge Street was enclosed. It remained a small town with probably 150-180 burgages-of which about half could have been accommodated within the castle walls.
After Owain Glyndwr's attacks of 1403 and 1405 it was decided to extend the castle walls, and Henry V gave a grant of £20.00 towards the cost. Two streams formed a natural moat around the west and north walls , with a marsh in the John Street area which was not drained until the 17th Century. The castle wall ran from eastwards from the south-east tower of the castle above Dan-y-banc , then north-west crossing Spilman and King Street. The wall then went south-west down Wood's Row, passing through the lower end of Jackson's lane as far as the Dark gate in Lower Guildhall Square. It then went south following the former stream now covered by Blue St, then turning east to link with the South Tower of the castle. There were gates at the crossing of Spilman and King Street, at the end of Red Lion Yard leading into Chapel Street, at the bottom of Bridge and Quay Street.
In the 14th Century the population was probably 1000. In 1326 it was made a staple port with a license to deal in wool, pelts, leather, lead and tin and was an important vase for the emerging fishing industry. By the end of the 16th century the population had grown to 2250 with development taking place outside the walled area along Lammas street and Priory St. Carmarthen was the most populous town in Wales until the large scale urbaisation of the early 19th Century