By train. Carmarthen station is on the West Wales line, with direct trains from Swansea (50 minutes), Cardiff (90 minutes), London Paddington (just over 3 hours), and onward to Tenby, Pembroke Dock and Fishguard Harbour for the Rosslare ferry. The station is on the south bank of the Tywi, a 10-minute walk over the bridge to the town centre.
By car. Carmarthen sits on the A40 trunk road from Brecon and the English border (2 hours from Bristol), and the A48 from Swansea (50 minutes). The town has several large pay-and-display car parks within walking distance of the market and castle.
By bus. Carmarthen bus station is in the town centre and is served by frequent local buses across Carmarthenshire and Pembrokeshire, plus TrawsCymru long-distance services to Aberystwyth (T1) and Cardiff (T1C). The journey from Cardiff by bus takes about 3 hours and is significantly cheaper than the train.
There is no nearby airport. Cardiff Airport is 90 minutes east by car; Bristol is the main international hub at 2.5 hours.