Since it and every wall or line that came after proved useless (given its cost and time biild) what would have been a better alternative? Assume roughly the same cost, or man hours, or volume moved.
Not trying to be argumentative, but I have read at several places that to some degree at least the GWOC was successful. The nomadic tribes that China was defending against really depended on their horses, and while humans didn’t have any trouble in getting over the wall, it generally was able to keep the horses out. Made the interlopers a lot easier to track down and defeat.
I’m also of the opinion that TGWOC had its uses, mainly limited incursions, but not a blitzkrieg (ancient or modern.) For the latter, I believe having a strong standing army makes fortifications useful only in specific points. From what I can glean from English history, scattering several castles around a large area is good for subjugating an occupied area, not for defending against an organized army.
Military “spoiling” expeditions – quick, hard raids against the leadership of the enemy tribes – might have been effective. If you can keep them from unifying, you don’t have much to fear from them.
Assuming I was able to unite China, I would have invaded Mongolia and set up armed camps thete.