The people of Windsor were used to having King Henry III's eldest son come down from London with all the display and magnificence of Royalty; but there was one particular day when the townsfolk looked on in surprise, for they saw him riding through the streets in haste, followed by a hundred armoured knights, and their esquires, and with them a great guard of Flemish soldiers. He was bringing with him his wife, a thousand marks in gold, and his mother's costly jewels, which he had taken by force from the Temple in London. Once within the Castle, he shouted his orders. Instantly the warders thrust the gates together, dropped the portcullis and ran up the Royal banner. Then he called out the garrison and, with these and the foreign mercenaries he had brought with him, he was ready to hold the castle against any who might attempt its capture.

This was in July 1263, during the troubled times immediately prior to Simon de Montfort's Baronial War against King Henry. The opposition leader had newly returned to England and was imposing himself on the King's Council by riding on a wave of anti-alien feeling in the country. Prince Edward had planned to be one step ahead of his father's enemies by securing the great fortress - which, according to previous arrangements, should have been in the barons' hands - before De Montfort broke out into open rebellion. In the meantime, however, the barons compelled the King to send orders to Windsor, telling the prince to surrender the Castle. The King's messengers came to Windsor, accompanied by a great force of armed men under the command of some of the leading nobles, including De Montfort himself. After only a short siege, whilst Edward debated how to proceed, the prince yielded to his father's orders on 26th. The foreign soldiers marched out of the Castle, taking with them their horses and their arms, as Edward had demanded. A strong force of barons and their retainers met them on the Castle Hill, and riding with them through Runnymede, and thence to the coast, did not suffer them to pass out of their sight, night or day, until they were embarked in some ships that were waiting to carry them back to France.