Offa became king of Mercia in 757. Over the next few years he extended his domination over Kent, Sussex, Wessex and East Anglia. Unable to defeat the Welsh, he built the great earthwork known as Offa's Dyke (784-86) stretching for 70 miles along the Welsh border. Offa introduced the silver penny which was to remain the basis of English currency until the 13th century. Offa died in 796.