Offa's Dyke separated the Anglian kingdom of Mercia from the Welsh Kingdom of Powys. King Offa of Mercia built the structure as a defence against attacks from the Welsh.
King Offa is recognised as one of the leading rulers of the Anglo-Saxon era. To have built such an embankment would have taken many years and much organisation. It would still have been under construction in 800 AD and is reputed to have stretched from sea to sea.
A good place to view Offa's Dyke and find out about its history is Knighton which is on the border between Shropshire and Powys. There is a dedicated visitor centre here and some of the best remains of the Offa's Dyke.
More details can be found at: www.offasdyke.demon.co.uk/