It is still uncertain whether the Dyke was built in one go or whether its course linked up a series of pre-existing barriers built across ridge-top routes and river-valley trackways.It is also hard to date the Dyke accurately, due to the difficulty in finding material incorporated within or under it that belongs exactly to the period of its construction.
This was exactly what the Franks were doing in continental Europe with the creation of the frontiers of the emergent ‘Carolingian Empire’, replete with customs-points.
The location of particularly complex stretches of the Dyke may correspond to notably sensitive ‘political places’ along the frontier where centres of Welsh power, such as may have existed on the later sites of Powis and Montgomery castles, may have been confronted by the earthwork.
What makes a simple earth ditch-and-bank dug along the frontier between Wales and England 1,200 years ago potentially fascinating to any historically-minded person living in Britain in the 21st-century?
For one thing, its length: the earthwork itself covers a distance of more than 80 miles (129km) and it ranges across a former frontier that spanned 115 miles (185km) – the latter extending from the shores of the Dee estuary in the north to Severn Estuary near Chepstow in the south.
This was carried out across a terrain replete with high moorland, steep-sided valleys, river-crossings and hilltops along what has become the borderland between England and Wales. Firstly, it served as a deterrent: it demonstrated the might of Mercian mobilisation – any kingdom with the resources to create such a massive work could surely crush any would-be invaders.
Secondly, the Dyke bolstered Mercia’s standing as a European power.By laying down the origins of conscripted armies; economic power through trade and a regulated currency; a court culture in which queens as well as kings held sway; and the first formal ‘diplomatic missions’ (such as the one in 789 that failed to successfully negotiate an alliance between the dominant power of Europe – Francia – and Mercia), Offa’s kingdom provided a first glimpse of the role a newly-assertive Britain could play in a wider international setting.And it involved the attempt to forge a ‘new relationship’ with Europe that is arguably just as elusive today as it was all those centuries ago.Offa and Coenwulf did not see themselves as ‘kings of the English’, however, as some past Anglo-Saxon historians have suggested.They saw themselves, rather, as ‘emperors of Mercia’.In c895 AD Bishop Asser, with the benefit of hindsight and in the setting of Alfred’s newly-ascendant West Saxon kingdom in the late ninth-century, saw Offa’s Dyke fundamentally as a vainglorious exercise by an unscrupulous and ruthless king.