Carmarthen Castle is a fragment by the standards of the great Welsh castles, but the 14th-century gatehouse and the precinct walk give a sense of how the town defended itself for four hundred years. Entry is free and the inner court hosts open-air events in summer.
The Carmarthen Market is the genuine town heart — a Victorian hall packed with stalls every day of the week and worth at least an hour, both for shopping and for the texture of west Wales life. The Friday farmers' section is especially good. St Peter's Church, the medieval parish church at the top of King Street, holds the tomb of Sir Thomas Picton, the Welsh general killed at Waterloo.
East of the centre, the site of Carmarthen Priory marks where the famous Black Book of Carmarthen was compiled, and the Moridunum Roman amphitheatre — the only visible Roman site in west Wales — sits in a small grassed enclosure ten minutes' walk further out. The Picton Monument on the road west out of town is a 25-metre obelisk visible for miles around.
The Carmarthen Museum at Abergwili, three miles east, is the regional museum of Carmarthenshire, housed in the former palace of the Bishops of St David's. It is one of the better small county museums in Wales. The National Botanic Garden of Wales is a 25-minute drive east at Llanarthney and warrants at least half a day.