Meanwhile the Norman fortresses of Albemarle, St. Valery, and others, were obtained possession of by various means, and were held in the name of King William; and Conan, a powerful burgess of Rouen, had entered into the conspiracy, and engaged to betray the capital into the hands of a lieutenant of Rufus. Robert at length was roused to the dangers which surrounded him, but finding himself without money to raise troops, he applied to Philip I., of France, for assistance. Philip responded to the call, and advanced with an army to the borders of Normandy: but Rufus sent him a sum of money as a bribe, and the French king returned at once to his own country.
Robert appealed to his brother Henry, whom he had placed some time before in possession of a portion of the Norman duchy, in return for a sum of £3,000 which Henry had advanced. Since that time frequent quarrels had occurred between them, and it is related that, on one occasion, Henry was arrested by the duke's orders, and kept for a short time in prison. However, on receiving Robert's request for succour, Henry came to Rouen, and rendered his brother important assistance. Reginald de Warrenne, the lieutenant of Rufus, was driven back and compelled to retreat: the burgess Conan was taken prisoner, and pushed by Prince Henry, with deliberate cruelty, through the window of a high tower in the cathedral.
Early in the year 1091, the Red King landed an English army in Normandy, and advanced into the country. Robert again applied to Philip of France who exerted himself to arrange a treaty of peace between the two brothers. By the provisions of this treaty, which was signed at Caen, the lands of Eu, Albemarle, Fécamp, and others, were assigned to Rufus; and it was agreed that no further attempt should be made by Robert upon the English throne. Robert was to be aided to conquer the districts of Maine and portions of Henry's territory in place of those which he resigned in Normandy, and William engaged to pardon those barons who had defended his brother's cause, and to restore to them their titles and lands. The barons of the two factions agreed that if the king survived the duke, he was to have possession of Normandy; and if the duke outlived the king, he should receive the English crown. This treaty was signed by twelve barons on each side, who swore to maintain its provisions.