Chepstow Castle (Welsh: Cas-gwent), located in Chepstow in Wales, on top of cliffs overlooking the River Wye, is the oldest surviving stone fortification in Britain. It was built under the instructions of the Norman lord William fitzOsbern from 1067, and was the southernmost of a chain of castles built along the English-Welsh border.
The speed with which William the Conqueror committed to the creation of a castle in Chepstow is testament to its strategic importance. At the time, the kingdoms in the area were independent of the English crown and the castle in Chepstow provided a way to suppress the Welsh from attacking Gloucestershire. It controlled an important crossing point on the River Wye, itself a major artery of communications to Monmouth and Hereford. The precipitous limestone cliffs beside the river, and a valley (the Dell) to the landward side, afforded an excellent defensive location.