Check your own (!!!)
That the Romans could build 80 miles of dressed stone fortification, 15ft high and 9ft wide, over crags and bogs with a small fort every mile, is indeed a marvel. It was one of Rome’s most expensive projects.
Yet I often ask visitors as they marvel: did it work? The answer is no. The Roman garrison was too strung out to defend the whole thing at once. Within 30 years it had been successfully attacked by the barbarians; within 40 it had been abandoned for a new wall inScotland;
http://www.rationaloptimist.com/blog/hadrians-wall-was-a-marvellous-mistake-so-is-hs2/
How did Genghis Khan get passed the Great Wall of China?In the following decades, the Mongols regularly found ways around or through the Wall. In 1279, the Mongols—under the leadership of Kublai Khan, Genghis' grandson—defeated the Song and ruled all of China for the next 100 years (the Yuan Dynasty).
