Along about 784, Offa, King of Mercia, decided that he had had enough of the Celtic invasions from Wales. Cant really blame him, really. They had poured across the borders in 760, 777, 778, and 784, trying to reclaim land Offa had taken from them.
(Theres the rub: Offa was having to defend land he had taken away in the first place.)
Anyway, he had taken this land and didnt feel like giving it back. So he decided to do something about it. Rather than putting everything he had into another invasion into Wales, he hit upon the idea of building a wall to keep the Welsh out. He had surely heard about or even seen the great Roman walls in the north (Hadrians Wall and the Antonine Wall). He had surely heard about or even seen how well those wallsproperly mannedkept the invaders out and the defenders happy.
So, Offa decided to have his own defensive structure built. It was a massive undertaking. At its greatest length, it extended 140 miles, from the River Dee to the River Wye. It was studded at various distances with fortifications.
Offa’s Dyke: Moving Earth and EarthHowever, it wasnt really a wall. Rather, it was more of a ditch, backed by earthworks. The effect on an invader would be tremendous: He would have to run down through a ditch, then immediately scale 20 feet up an earthwork, all the while being in range of arrows, stones, burning logs, and anything else the defender chose to lob at him. And he couldnt exactly go around, since the bloody thing was 140 miles long.
This defensive structure came to be called Offas Dyke, even though it wasnt much of a dyke (other than the fact that it stemmed the tide of Welsh invasions). It took 12 years to build. It was an ongoing project, and it wasnt exactly solid through and through. Some spots were barely defensible, especially when the rivers that made them easily defensible in when the water level was high suddenly had much lower water levels, allowing easy crossings. Other spots were virtually impregnable.
It was a great achievement of unity and a testament to the power of Offa that he could motivate his people to keep building for so long. After his death, in 796, construction all but stopped.
Much of the Dyke can still be seen today. It still forms rather much of the boundary between England and Wales.