Built by Offa, King of Mercia (a.d. 757 796), the impressive earthwork known as Offas Dike formed a boundary, albeit discontinuous, between England and Wales. One of the most remarkable structures in Britain, it runs 177 miles (280 kilometers) 7.5 miles (12 kilometers) longer than Hadrians Wall from the Dee Estuary in the north to the River Wye in the south. It is now generally agreed that the dike was not so much a fortification as a substantial line of demarcation. No earlier Anglo-Saxon king had unified southern England as Offa did, and with unity came power and wealth. He also formed ties with rulers across the channel and was accepted as an equal by Charlemagne, king of the Franks, with whom he entered a commercial treaty in 796. Even Pope Adrian I treated him with great respect.
Some have suggested that the dike was built to give Mercia, command of the approaches to the English lowlands, but parts of it rise as much as 1,300 feet (400 meters) above sea level. The boundary it marked was hardly precise during Offas reign there were English communities to the west of it and Welsh communities to the east. Be that as it may, the dikes very presence made a strong statement about separation. There is an apocryphal story that it was [once] customary for the English to cut off the ears of every Welshman who was found to the east of the dike, and for the Welsh to hang every Englishman whom they found to the west of it.
The dike consists of an earth bank in places 20 feet (6 meters) high with a 12-foot-deep (3.6-meter) ditch on the western side sometimes their combined width was 60 feet (18 meters). Natural features seemed to be used wherever practicable, but for the most part an earth embankment was built a total of 80 miles (130 kilometers). Elsewhere it was discontinuous, giving rise to the speculation that, having been initiated late in Offas reign, it was never finished. On the other hand, perhaps in those locations, the local topography served the same purpose. Welsh historian John Davies writes that Offas Dike was perhaps the most striking man-made boundary in the whole of Western Europe. Thousands of workers must have toiled to build it, evidence of Offas resources and the integrity of his kingdom. In places the straightness of its line for kilometers is evidence of the technical skills of its builders. No written records about the project survive. After 1066 the Norman invaders saw the value of the dike for defense, and many castles and abbeys of early date stand in its shadow on the eastern side.
Offas Dike is a reminder of persistent Welsh-English antipathy. Although there is everywhere in Britain a challenge to the myth of a united kingdom, lately evidenced by the Scottish and Welsh elections of 1999, the signs of cultural disintegration are nowhere stronger than in Wales. That is expressed even in language differences. As someone has commented: for the source, we must look to the mid eighth century, when a long ditch was constructed, flanking a high earthen rampart that divided [the Celts from the Saxons] and which even today marks the boundary between those who consider themselves Welsh [and] those who consider themselves English.