Carmarthen (kərmärˈħən) [key], Welsh Caerfyrddin, town (1981 pop. 13,860), Carmarthenshire, S Wales, on the Tywi (Towy) River. It is a port for small vessels, a transportation hub, a cattle market, and a dairy center. In the Middle Ages, Carmarthen was an important wool port. Its old castle (now in ruins) was the headquarters of Welsh chieftains. Carmarthen's parish church of St. Peter (14th cent.) is also noteworthy. A campus of the Univ. of Wales Trinity St. David is there.