Chepstow Castle (Welsh: Cas-gwent), located in Chepstow, Monmouthshire in Wales, on top of cliffs overlooking the River Wye, is the oldest surviving post-Roman stone fortification in Britain. Its construction was begun under the instruction of the Norman Lord William fitzOsbern, soon made Earl of Hereford, from 1067, and it was the southernmost of a chain of castles built along the English–Welsh border in the Welsh Marches.