At the same time, however, he was not blind to the possibilities of papal interference in domestic matters, and of the danger of conflict between the crown and the recently-strengthened clerical order. To guard against them he laid down three general rules: (1) that no one should be recognized as pope in England till he had himself taken cognizance of the papal election, and that no papal letters should be brought into the realm without his leave; (2) that no decisions of the English ecclesiastical synods should be held valid till he had examined and sanctioned them; (3) that none of his barons or ministers should be excommunicated unless he approved of such punishment being inflicted on them. These rules seem to argue a deeply rooted distrust of the possible encroachments of the papacy on the power of the state. The question of ecclesiastic patronage, which was to be the source of the first great quarrel between the crown and the church in the next generation, is not touched upon. William retained in his own hands the choice of bishops and abbots, and Alexander II. and Gregory VII. seem to have made no objection to his doing so, in spite of the claim that free election was the only canonical way of filling vacancies. The Conqueror was allowed for his lifetime to do as he pleased, since he was recognized as a true friend of the church. But the question was only deferred and not settled. William's later reign. The political history of William's later years is unimportant; his main energy was absorbed in the task of holding down and organizing his new kingdom. His rather precarious conquest of the county of Maine, his long quarrels with Philip I. of France, who suborned against him his undutiful and rebellious eldest son Robert, his negotiation with Flanders and Germany, deserve no more than a mention. It is more necessary to point out that he reasserted on at least one occasion (when King Malcolm Canmore did him homage) the old suzerainty of the English kings over Scotland. He also began that encroachment on the borders of Wales which was to continue with small interruptions for the next two centuries. The advance was begun by his great vassals, the earls of Chester, Shrewsbury and Hereford, all of whom occupied new districts on the edge of the mountains of Powys and Gwynedd. William himself led an expedition as far as St Davids in 1081, and founded Cardiff Castle to mark the boundary of his realm north of the Bristol Channel.