In order to strengthen himself still further, and at the same time to erect a bulwark between him and England, Llewelyn cemented his alliance with the powerful lords of the Marches by giving one of his daughters in marriage to Reginald de Braose, and the other to Ralph Mortimer. This lady Gladys, who married Mortimer, became the ancestress of Elizabeth of York, the mother of Henry VIII. Thus in the veins of the greatest of the Tudors there flowed some of the blood of the ancient royal house of Cunedda. {77} In the next period of his reign, between 1215 and 1226, we find Llewelyn at war with the Marshalls, the able and warlike Earls of Pembroke. William Marshall was as able a statesman as Llewelyn himself. His aim was to put an end to the turmoil into which England had been plunged by the struggle between king and barons. He was in favour of the Charter; but at the same time it seemed to him that the supremacy of a king was to be preferred to the lawless self-seeking of the great earls. He believed that the welfare of England demanded the existence of a strong central Government; and in this he was unquestionably right. In truth, Marshall was striving to achieve in England the very same thing that Llewelyn had been striving to achieve in Wales. With the death of John, and the accession of the innocent and untried Henry III, the power of the barons began to decline. Unfortunately the great Marshall died in 1219; and a vigorous war broke out between his son and Llewelyn. After some years, however, the younger Marshall lost the favour of the king; and Llewelyn, always quick to adjust his policy to a changed situation, at once concluded an alliance with him. Against such an alliance {78} nothing could stand in Wales. Every castle in the country, with the solitary exception of Carmarthen, fell into their hands, and the king's army was defeated in a great battle at Grosmont.