Blaenllyfni is a ruinous castle near the village of Bwlch in southern Powys, Wales. It was at the centre of the Fitz Herbert barony of Talgarth, which was created as a third part of the Brecon lordship in 1208. The castle was therefore most likely constructed in 1208–1215.
It was sacked by Prince Llywelyn ab Iorwerth and Richard Marshall in October 1233. Soon rebuilt, it was apparently taken by Llywelyn ap Gruffydd in 1262, but by September 1273 it had been retaken by Reginald fitz Peter fitz Herbert. Around 1322 the castle was seized by the Crown and given to the Despensers. At the time of the execution of Hugh Despenser the Younger in 1326, it was nearly ruinous, probably having never been adequately repaired after the siege by Llywelyn ap Gruffydd.
The castle is on private grounds. There are remains of a keep and several towers.
Remfry, P. M. The Castles of Breconshire. Logaston, 1999.