A Dead End

Obviously, the ancient Chinese didn't expect that the great wall would actually be able to keep the invading Mongolian hoardes out of China. They knew that the occasional Mongol would make it through the gaps; it's fairly easy to get around the end of the wall, even if it does end on a 'cliff'. But they also had to have known that once the occasional Mongol made it through, he'd be assailed by all of the tourists and shop vendors, and would end up loaded down with junk from the vendors with any money that wasn't taken by the pick-pockets.

Of course, then the invading Mongol would have to bring his family and get their picture taken (by a thief that would make of with their camera), leaving the entire family depressed and unwilling to invade any further.

No, China wasn't really protected by the great "wall". China was protected by the greatest tourist trap in the world. These days, though, it probably traps more Chinese than invading Mongolians. But at least this explains the dead ends; the wall doesn't need to be continuous, and even keeps people /on/ the wall better when you've got to backtrack past the vendors that've already taken your shirt.