The land of Brycheiniog was conquered between 1070 and 1093. In 1070 William FitzOsbern, 1st Earl of Hereford invaded the kingdom and defeated three kings of South Wales, but no king of Brycheiniog. King Bleddyn of Brycheiniog, who was alleged to be ruling at the time of the Norman conquest and was said to have been defeated by Bernard de Neufmarché, appears in no historical source before the fifteenth century. By 1088 Bernard de Neufmarché mentioned 'all the tithes of his lordship which he had in Brycheiniog in the woods and plains' as well as Glasbury. This suggests that he already thought himself lord of Brycheiniog. In April 1093 he defeated and killed the king of Deheubarth, Rhys ap Tewdwr while he was building a castle at Brecon. The Welsh Annales clearly state that Rhys was killed 'by the French who were inhabiting Brycheiniog'. In other words, the Normans were already living there and the kingdom had already been destroyed. The kingdom was subsumed within the lordship of Brecon, ruled by Bernard's descendants.

By 1136 an opportunity arose for the Welsh to recover lands lost to the Marcher lords after Stephen de Blois had displaced his cousin Empress Matilda from succeeding her father to the English throne the previous year, sparking the Anarchy in England. The usurpation and conflict it caused eroded central authority in England. The revolt began in south Wales, as Hywel ap Maredudd, lord of Brycheiniog (Brecknockshire), gathered his men and marched to Gower, defeating the Norman and English colonists there in the Battle of Llwchwr. Inspired by Hywel of Brycheiniog's success, Gruffydd ap Rhys, Prince of Deheubarth, hastened to meet with Gruffydd I of Gwynedd, his father-in-law, to enlist his aid in the revolt. However, with Gruffydd ap Rhys' absence the Normans increased their incursions into Deheubarth. Gruffydd ap Rhys' wife Gwenllian, Princess of Deheubarth, gathered a host for the defense of her country.

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