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.
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.
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.
BUILTH, or LLANVAIR-YN-MUALLT, a market-town and parish, the head of a union, and anciently a borough, in the hundred of Builth, county of Brecknock, South Wales, 17 miles (N.) from Brecknock, and 170 (W. W.) from London; containing 1203 inhabitants.
The lordship of Builth descended, together with the lordships of Brecknock and Hay (the latter in right of his mother), to William, son of Philip de Breos, upon whose subsequent attainder they became forfeited to the crown.
Llewelyn fled towards Builth, crossing the bridge over the Wye, which he caused to be demolished, before the arrival of his pursuers, who were, consequently, compelled to return to a ford eight miles lower down on the river, where they effected a passage.