I've just come back from a section of the wall called Mutianyu, so maybe other sections are different. But the wall seemed a little unimpressive compared to the pretty impregnable mountains it was built on. But I feel I might be missing something.
So my question is, was it actually worth spending all those resources to build large parts of the wall?
The Great Wall, for its pure military purposes, also utilize some psychological warfare elements.
That is, the ancient Chinese wanted to make a statement to the "barbarians": We can build this massive long wall on top of these mountains, you don't want to cross this wall. If we can build this wall, we can fight you, and you won't win.
Did it work? Was it worth it?
For the most part, I think it did work. But like most visible defensive structures, it will be continually tested by potential attackers. And sooner or later, some attacker might succeed.