 Closely following the River Wye, Offa's Dyke was built in the 8th century under King Offa to mark out the boundary between England and Wales and is, today, the longest archaeological monument in Britain.Offa's Dyke Path long distance footpath today traces the route through the Wye valley on the English bank, while the Wye Valley Walk follows the Welsh bank.When the Normans conquered the region in the 11th century they immediately built major castles at Chepstow and Monmouth to defend the territory.