Owen, finding that Harry Percy and the young prince were too strong to be attacked, now fell with all his force on South Wales, harrying the land of the English and of such Welsh as would not join him. Then he abruptly turned to the Severn valley, burnt Montgomery, and was only stopped under the red walls of the castle of Percy at Welshpool. Now all Wales was in insurrection, and everywhere Owen was regarded as one who would deliver the Cymry from their hereditary oppressors. The rapid progress of his army spread terror along the Marches, and messengers on swift horses galloped to London to announce to the King that unless succour were sent his castles would fall. In October, 1401, King Henry and the Prince of Wales entered the Principality at the head of a huge army, and pushed on to Bangor, only to find that the Welsh had retreated to the mountains, carrying off with them all their goods. The King passed along the coast to the abbey of Strata Florida in Cardiganshire, which he gave up to pillage and fire. Having succeeded in capturing about a thousand Welsh children without having fought a battle, Henry ingloriously withdrew. About this time, moreover, Owen succeeded in getting hold of his great enemy Lord Grey de Ruthin, and sent him to his tower of Dolbadarn, there to languish until he could raise the heavy ransom which Owen, who was sorely in want of money, demanded for his release. Henry Percy, unable to obtain payment for his services in Wales, and reimbursement for large sums laid out by himself in the King’s service, threw up his charge and retired to Northumbria to fight the Scots. In May, 1402, Owen Glyndwr attacked the Welsh territories of young Edmund Earl of March, who, with his younger brother Roger, was held in custody by the King, on account of his having been acknowledged by Parliament to be the lineal heir to King Richard. Sir Edmund Mortimer, their uncle and guardian, hastened to protect the lands, assisted by the other Marchers. They met on the border in a narrow valley at Pilleth, near Knighton, and during the battle the Welsh tenants went over in a body to the side of Glyndwr. Eleven hundred men were killed, and Mortimer was captured. Then ensued the dispute between Harry Hotspur and King Henry which has been immortalised by Shakespeare. Henry Percy’s wife was the sister of Sir Edmund Mortimer, and he was urgent for the ransoming of the captive. But King Henry was in sore straits for money, and he was, moreover, not particularly desirous to have the uncle of the true heir to the throne at large. What he did was to lead an army a third time into Wales, whilst a second was placed under the command of the Prince of Wales, and a third under that of the Earl of Warwick.