consistent with the British victory at Badon Hill , attributed to Arthur by Nennius . [ 45 ] This tenacious survival by the Romano-Britons and their descendants in the western kingdoms was to become the foundation of what we now know as Wales. With the loss of the lowlands, England's kingdoms of Mercia and Northumbria , and later Wessex , wrestled with Powys, Gwent and Gwynedd to define the frontier between the two peoples. Having lost much of what is now the West Midlands to Mercia in the 6th and early 7th centuries, a resurgent late-seventh-century Powys checked Mercian advancement. Aethelbald of Mercia , looking to defend recently acquired lands, had built Wat's Dyke . According to John Davies , this endeavour may have been with Powys king Elisedd ap Gwylog 's own agreement, however, for this boundary, extending north from the valley of the River Severn to the Dee estuary, gave Oswestry to Powys. [ 46 ] Another theory, after carbon dating placed the dyke's existence 300 years earlier, is that it may have been built by the post-Roman rulers of Wroxeter . [ 47 ] King Offa of Mercia seems to have continued this consultative initiative when he created a larger earthwork, now known as Offa's Dyke ( Clawdd Offa ). Davies wrote of Cyril Fox 's study of Offa's Dyke: "In the planning of it, there was a degree of consultation with the kings of Powys and Gwent. On the Long Mountain near Trelystan, the dyke veers