"sea fort") was the civitas capital of the Demetae tribe in Roman Wales and was recorded by Ptolemy and in the Antonine Itinerary.

The initial fort is believed to date from about AD 75, possibly replacing the hillfort on Merlin's Hill.

[1] A map from 1723 by William Stukeley places Mori dunum (Caermarthen) at the western extremity of the network of Roman roads in Southern Wales.

[2] A street-grid was laid out in the town and a public bath house built, and possibly a mansio.

A turf bank and ditch was erected around the town in the mid-2nd century and a stone wall added some time later.