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 Skenfrith (in Wales), towards Monmouth, looping eastwards to include the town itself and a surrounding area within Wales. At Redbrook, the boundary again reaches the Wye, and follows the river southwards, past Tintern and Chepstow on the Welsh side, to its confluence with the Severn at the Severn Bridge. The boundary then continues down the Severn estuary towards the Bristol Channel, with the small island of Flat Holm being administered as part of Wales and the neighbouring island of Steep Holm as part of England. The boundary passes between Flintshire, Wrexham County Borough, Powys, and Monmouthshire in Wales and Cheshire