What is "arwystli"

Arwystli was a cantref in mid Wales in mediaeval times, located in the headland of the River Severn. It was chiefly associated with the Kingdom of Powys, but was heavily disputed between Powys, Gwynedd, and the Norman Marcher Lords for hundreds of years, and was the scene of many skirmishes between those groups. Like many other cantrefs and subdivisions, it was divided up by the Laws in Wales Acts in the 16th century.