A barrier is a very simple thing to do. The technology is well tested. The Chinese had success with it, as did Hadrian
They did, eh? How did that work out for them exactly? Wikipedia says:
The Manchus were finally able to cross the Great Wall in 1644, when the gates of Shanhai Pass were opened by Wu Sangui, a corrupt Ming border general, after being bribed. The Manchus quickly seized Beijing, and defeated the remaining Ming resistance, to establish the Qing Dynasty.
So success = the complete overthrow of your country and the establishment of a 300 year dynasty. Gotcha. Well, I guess we have some interesting times ahead of us.