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Offa's Dyke is a huge linear earthwork structure which runs roughly along the current border between England and Wales. Construction is believed to have started in 785 and continued for several years.

One of the great engineering achievements in British history, the dyke was created on the orders of Offa, the powerful ruler of the Anglian kingdom of Mercia, between 757 and 796.

Offa controlled large swathes of land in England, and enjoyed alliances with Northumbria and Wessex. He was also adept at international trade and diplomacy, and established the penny as England's standard monetary unit.

The dyke was intended to place a boundary between his domain and the Welsh kingdom of Powys. It consisted of a ditch and rampart and was over 20m wide and 8m high, though natural levelling, farming and land development has reduced this somewhat in the intervening years.