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. The castle is on private grounds. There are remains of a keep and several towers.