Builth, Or Builth Wells, a market town of Brecknockshire, Wales. Pop. of urban district (1901), 1805. It has a station on the Cambrian line between Moat Lane and Brecon, and two others (high and low levels) at Builth Road about 1&FRAC34; m. distant where the London & North-Western and the Cambrian cross one another. It is pleasantly situated in the upper valley of the Wye, in a bend of the river on its right bank below the confluence of its tributary the Irfon. During the summer it is a place of considerable resort for the sake of its waters - saline, chalybeate and sulphur - and it possesses the usual accessories of pump-rooms, baths and a recreation ground. The scenery of the Wye valley, including a succession of rapids just above the town, also attracts many tourists. The town is an important agricultural centre, its fairs for sheep and ponies in particular being well attended.
The town, called in Welsh Llanfair (yn) Muallt, i.e. St Mary's in Builth, took its name from the ancient territorial division of Buallt in which it is situated, which was, according to Nennius, an independent principality in the beginning of the 9th century, and later a cantrev, corresponding to the modern hundred of Builth. Towards the end of the 11th century, when the tide of Norman invasion swept upwards along the Wye valley, the district became a lordship marcher annexed to that of Brecknock, but was again severed from it on the death of William de Breos, when his daughter Matilda brought it to her husband, Roger Mortimer of Wigmore. Its castle, built probably in Newmarch's time, or shortly after, was the most advanced outpost of the invaders in a wild part of Wales where the tendency to revolt was always strong. It was destroyed in 1260 by Llewellyn ab Gruffydd, prince of Wales, with the supposed connivance of Mortimer, but its site was reoccupied by the earl of Lincoln in 1277, and a new castle at once erected.
It was with the expectation that he might, with local aid, seize the castle, that Llewellyn invaded this district in December 1282, when he was surprised and killed by Stephen de Frankton in a ravine called Cwm Llewellyn on the left bank of the Irfon, 2&FRAC12; m. from the town. According to local tradition he was buried at Cefn-y-bedd ("the ridge of the grave") close by, but it is more likely that his headless trunk was taken to Abbey Cwmhir. No other important event was associated with the castle, of which not a stone is now standing. The lordship remained in the marches till the Act of Union 1536, when it was grouped with a number of others so as to form the shire of Brecknock. The town was governed by a local board from 1866 until the establishment of an urban district council in 1894; the urban district was then made conterminous with the civil parish, and in 1898 it was re-named Builth Wells.