Cantref and small kingdom in south central Wales, later the Marcher lordship of BUILTH; the present town of Builth Wells, with the ruined castle of the lordship, stands on the River Wye and on the eastern border of the cantref. It lies south of Llandrindod Wells and north of Brecon.
The Kingdom was too small to be able to withstand powerful neighbours and it became part of the south Wales Kingdom of Deheubarth formed by Hywel Dda in the first half of the 10th century. In the 1050s it was included in the dominions of Llewelyn ap Gruffydd.
For much of the 11th century and until 1230 Builth was held by the great Marcher family of Braose (Breos). With their extinction, possession passed to Llewelyn the Great, Prince of Gwynedd, who died in 1240, though two years later Builth was in the hands of the English King.
In 1256 Llewelyn the Last recovered Buellt for Gwynedd. Control of it brought the great power of northwest Wales into the river valley that led into the south east and created a barrier between the lands of western Wales and the Marchlands in Herefordshire and Shropshire. In 1277 Llewelyn came up against the legal pedantries and military vigour of Edward I and lost Buellt. When he made his final bid for power in 1282, it was in this strategically important region, close to Builth Wells, that he met his death.
From 1277 the lordship of Builth was either in the hands of the King or the Prince of Wales. In 1536 it became the north of the new county of Breconshire.