From the 6th century, most of Wales became christianized through the efforts of missionaries, amongst them St David and St Teilo. At this time, there was encroachment from Anglo–Saxon settlers, following the valley of the River Severn into mid-Wales. Anglo-Saxon dominance within England was confirmed at the Battle of Chester (616), where Northumbria triumphed over a confederation of Welsh tribes led by Powys. Later, a Welsh king of the Borders, Cynddylan, became an ally of Penda of Mercia in his wars with Northumbria. By the 8th century, two kings of Mercia built earthworks, Wat’s Dyke and Offa’s Dyke, to protect against Welsh incursions. It was not until the 850s that Rhodri the Great of Gwynedd achieved control of the bulk of Wales. Thereafter, Welsh inheritance customs led to the kingdom being repeatedly subdivided.
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