This is coolbert:
Thanks here to Al Nofi, CIC # 173, through StrategyPage.
“'Offa's Dyke,' a defensive barrier intended to serve as an obstacle to Welch raids into his kingdom." A moat on the Welch side, a "wall" on the Anglo-Saxon side.
This was a new one on me. Offa's Dyke. Equal or exceeds Hadrian's Wall [Roman Emperor Hadrian]. Still exists as a tourist attraction and hiking trail. NOT of course the same as the Great Wall of China. Evidence however, that even during the "Dark Ages", with sound government and administration, impressive feats of engineering were possible. As with all barriers, DID NOT stop all movement, but did slow down and deter to a large extent. Did the job intended.
Hail Offa! Hail Mercia!