About AD 75, a Roman fort was established at the site of modern Carmarthen during the successful campaign of Julius Frontinus to conquer Wales. A civilian settlement grew up around the fort, but was abandoned about AD 120, to be succeeded by a small town. This covered about 13.2 hectares and was the civitas(self-governing capital) of the Demetae tribe. Its Roman name means the ‘Sea Fort’ and it is one of only two tribal capitals the Romans established in Wales.

From the late 2nd century, like many other towns at the time, Moridunum was given earth ramparts and then stone defences. Imperial permission had to be granted for these, and Moridunum may have been given its grant of self-government then. The town had a regular street plan and public buildings, including a bathhouse and what may be a mansio (official inn). A fine town house with mosaics and a hypocaust (central heating system) has been found, as well as traces of ironworking and grain processing. There was an amphitheatre outside the walls.

Moridunum was occupied until the late 4th century, when it probably suffered from the general breakdown in civic order of the period and the effects of Saxon and Irish raids on Britain.