Edward had organised a large full-time army with which he overran all North Wales as far west as the Conwy River, and a fleet with which he captured Anglesey, depriving the Welsh of much of their grain.

Over the five years which followed, there was continued tension between Llywelyn and Edward over various lawsuits, and increasing unrest between the Welsh people and their English administrators in the newly transferred areas.

The revolt was actually begun in 1282 by Llywelyn's brother Dafydd, who had sided with Edward five years earlier but now captured Hawarden Castle and slaughtered its garrison.

On 6 November, Edward's lieutenant in Anglesey, Luke de Tany, launched a premature attack across a bridge of boats which spanned the Menai Strait.

However, three of them, Roger l'Estrange, John Giffard and Edmund Mortimer were staunch supporters of Edward, and were in the field in mid-Wales with an army consisting mainly of archers from Shropshire and some heavy cavalry, as well as troops donated to them by the Welsh lord Gruffydd ap Gwenwynwyn, who was an old enemy of Llywelyn.

On 11 December, Llywelyn's army occupied a hillside north of the Irfon River near the village of Cilmeri, placed to repel any attack from the south across Orewin Bridge.

This version of events was written in the north of England some fifty years later and has suspicious similarities with details about the Battle of Stirling Bridge in Scotland.

His army was immediately engaged in fierce battle during which a significant section of it was routed, causing Llywelyn and his eighteen retainers to become separated.