The battle resulted in heavy casualties for the Welsh, including two important commanders, Owain Glyndwr's brother and eldest son.

The battle began with a daring assault by Welsh forces, led by Gruffudd ab Owain Glyndŵr against Usk Castle.

Adam of Usk’s contemporary description gives a vivid picture of the attacks on the retreating Welsh: “there slew with fire and the edge of the sword many of them, and above all the Abbot of Llanthony, and they crushed them without ceasing, driving them through the monk's wood, where the said Griffin (Owain's son) was taken.”[4] According to the Scottish chronicler Walter Bower, Dafydd Gam, a committed enemy of the rebellion, a Welshman originally from Brecon but holding land at Llantilio Crossenny, Monmouthshire, played a significant part in the English victory.

It is perhaps significant that just two years earlier in 1403, Owain Glyndŵr had burnt the town of Usk to the ground, with loss of life and property so local people may not have favoured his cause.

The presence of such English leaders as Grey and Greyndor combined with men with excellent local knowledge and reputations as Dafydd Gam and John Oldcastle at the exact time of a Welsh attack does seem indicative if not conclusive of there being foreknowledge.

Another serious blow was the death of John ap Hywel, Abbot of the Llantarnam Cistercian monastery, a notable supporter of Glyndŵr who was killed during the battle as he ministered to the dying and wounded of both sides.

One history of the rebellion says of the battle that the defeat “suggest that the rashness of local initiatives was endangering the revolt as a whole.”[7] It certainly indicates the dangers of Owain's guerrilla warfare tactics when the enemy were prepared.

Defeat in the battle and the loss of many good men was to undermine the possibilities offered by the French troops that arrived later that year to support Glyndwr.

It is interesting to conjecture that with victory behind him at Usk and his brother still alive and south east Wales secure how Owain might have conducted himself at Abberley Hill - the battle that never was.