Roman Theatre Moridunum

Moridunum (meaning 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. The fort lasted until about 120, when the associated civilian vicus took over and the place became a town.

A street-grid was laid out in the town and a public bath house built, and possibly a mansio. The forum and basilica were probably under the most built-up area of the present town on the Cardo or main street. There were narrow shops fronting the streets, as well as evidence of metalworking. Large domestic homes of timber were rebuilt in stone in the late 3rd or early 4th century. A 1st/2nd century Romano-Celtic style temple has also been excavated. A turf bank and ditch was erected around the town in the mid-2nd century and a stone wall added some time later.

East of the old town is one of only seven surviving Roman amphitheatres in the United Kingdom. It was excavated in 1968. The arena itself is 46 by 27 meters. The circumference of the cavea seating area is 92 by 67 meters . It had stone walls and wooden seating and was much larger than would be expected for the size of the town.

The name became Caerfyrddin and was later anglicized to Carmarthen. Caerfyrddin may have been misinterpreted as 'Fort of Myrddin', hence the town's association with Merlin the Magician.