 Having lost much of what is now the West Midlands to Mercia in the 6th and early 7th centuries, a resurgent late-7th-century Powys checked Mercian advances.Aethelbald of Mercia, looking to defend recently acquired lands, had built Wat's Dyke.According to John Davies, this endeavour may have been with the agreement of Powys king Elisedd ap Gwylog, as this boundary, extending north from the valley of the River Severn to the Dee estuary, gave Oswestry to Powys.