It was part of the Welsh Revolt of 1400-1415 led by Owain Glyndŵr against English rule.

The settlers were reinforced by a large force of English soldiers and Flemish mercenaries.

[1] It is thought that Owain's force would have been made up mostly of archers mounted on hill ponies that would have been well suited for travelling across boggy or mountainous regions.

However, it is known that Glyndŵr's army was able to fight back these attackers (despite being outnumbered and on the low ground), killing 200, chasing the main force away and making prisoners of the rest.

The English army (being more heavily laden) would have had more trouble traversing the marshy ground of the valley.