A sudden uprising of the city forced the Empress to "ignominious" flight, leaving her baggage behind. She retreated to Oxford, and Matilda the queen entered the recovered city. Geoffrey de Mandeville at once brought his allegiance to the new market and obtained, it is probable, another advance of price and Henry of Winchester was easily persuaded to return to his brother's side. "Behold," says the historian of the Empress's party, "while she was thinking that she could immediately possess all England, everything changed." He adds that the change was her own fault, and in this he was right.[42] But Matilda was not ready to accept calmly so decided a reverse, nor to allow Winchester to remain in undisturbed possession of her enemies, and her brother Robert was not. They had been driven from London on June 24. At the end of July, with a strong force, they attacked the older capital city, took possession of a part of it, forced the bishop to flee, and began the siege of his castle. At once the leaders of Stephen's cause, encouraged by recent events, gathered against them. While the Empress besieged the bishop's men from within, she was herself besieged from without by superior forces. At last the danger of being cut off from all supplies forced her to retreat, and in the retreat Robert of Gloucester, protecting his sister's flight, was himself captured. This was a great stroke of fortune, because it balanced for practical purposes the capture of Stephen at the battle of Lincoln, and it at once suggested an even exchange. Negotiations were not altogether easy. Robert modestly insisted that he was not equal to a king, but the arrangement was too obvious to admit of failure, and the exchange was effected at the beginning of November. Since the middle of June the course of affairs had turned rapidly in favour of the king, but he was still far from having recovered the position of strength which he occupied before the landing of Matilda. Oxford was still in her hands, and so was a large part of the west of England. The Earl of Chester was still on her side, though he had signified his willingness to change sides if he were properly received. Stephen had yet before him a hard task in recovering his kingdom, and he never accomplished it. The war dragged on its slow length for more than ten years. Its dramatic period, however, was now ended. Only the story of Matilda's flight from Oxford enlivens the later narrative. Siege and skirmish, treason and counter-treason, fill up the passing months, but bring the end no nearer, until the entry of the young Henry on the scene lends a new element of interest and decision to the dull movement of events.