Chepstow Castle (Welsh: Cas-gwent) at Chepstow, Monmouthshire, Wales is the oldest surviving post-Roman stone fortification in Britain. Located above cliffs on the River Wye, construction began in 1067 under the instruction of the Norman Lord William fitzOsbern. Originally known as Striguil, it was the southernmost of a chain of castles built in the Welsh Marches, and with its attached lordship took the name of the adjoining market town in about the 14th century. The ruins were Grade I listed on 6 December 1950.