England–Wales border after Harold's death at the Battle of Hastings, Wales was again divided without a leader to resist the Normans. Immediately after the Norman conquest of England, King William installed one of his most trusted confidants, William FitzOsbern, as Earl of Hereford. By 1071 he had started the building of Chepstow Castle, the first castle in Britain built of stone, near the mouth of the Wye. It served as a base from which the Normans continued to expand westward into south Wales, establishing a castle at Caerleon and extinguishing the Welsh kingdom of Gwent. William also installed Roger de Montgomerie at
England–Wales border 1972. Whether Monmouthshire was part of Wales, or an English county treated for most purposes as though it were Welsh, was also settled by the 1972 Act, which included it in Wales. The modern boundary between Wales and England runs from the salt marshes of the Dee estuary adjoining the Wirral Peninsula, across reclaimed land to the River Dee at Saltney just west of Chester. It then loops south to include within England an area southwest of Chester, before rejoining the Dee, and then loops east of the river to include within Wales a large area known as Maelor, formerly