The background to this battle is the supposed rape and abduction of Princess Nest (ferch Rhys ap Tudur) by her kinsman Owain ap Cadwgan, heir to the Kingdom of Powys some seven years earlier.

At the instigation of Henry I of England, Nest his former mistress was married to Gerald de Windsor, the Norman castellan of Pembroke Castle. Struck by her beauty, Owain thought that Nest was too good for a Norman and at Christmas 1109 he attacked and burnt Gerald’s castle at Cilgerran and abducted his wife and children; Gerald escaping via the lavatory chute. Eventually Nest and her children were returned to Gerald.

Owain, a hothead and a militant was indiscriminate in his looting raids on Welsh, Flemings and Normans, exiling himself to Ireland when under threat. By 1111 Owain had become ruler of much of Powys. By 1116 he was assisting King Henry in the pursuit of Gruffydd ap Rhys, Prince of Deheubarth and brother of Nest after Gruffydd’s defeat at the battle of Llanfarian. Also in the pursuit was Gerald and an army of Flemish soldiers.

Brut y Tywysogion (Chronicle of the Princes) gives a clear description of the events that followed. Owain, returning to his old ways persued and caught sympathisers of Gruffydd ap Rhys who were taking their livestock to the safety of Carmarthen. Returning slowly along the River Tywi with the captured livestock and a force of 90 men he was spotted by the Flemings and they reminded Gerald of the hurt Owain had caused him……. and they persued him, and forthwith they came to the place where Owain was, and the spoil with him. And when Owain’s comrades saw the multitude coming after them, they said to Owain, ‘There is a multitude pursuing us, whom we have no power to encounter.’ And he replied to them and said, ‘Let there be no fear upon you without cause. Nothing can be done,’ said he, ‘with the Flemings.’ And then he attacked them boldly, and they too stood manfully; and with shooting on either side Owain was wounded till he was slain.

The site of the battle in near the present Nantgaredig Bridge over the River Tywi. Ystrad Rwnws is a later mediaeval corruption of Ystrad yr Ynys. The church to St Teilo at Llandeilo yr Ynys stood nearby on high ground above a marshy plain.