Not against the Mongolians per se – it protected them against their horses.
It bothered me for a really long time. As far as I could tell, you could simply scale the wall. There are places where it’s barely more than a few meters tall (sure, you have to climb a pretty steep hill first), but for a footman, it wouldn’t be a challenge at all.
The truth is though, what was dangerous wasn’t the Mongolians – it was Mongolians on horseback.
Dan Carlin does a fascinating job of explaining just how effective these forces were, and I definitely recommend you check out his Wrath of the Khans episodes. Essentially, hit and run tactics and a complete mastery of horseback riding, given that the Mongols essentially grew up on them, to an extent that they could shoot arrows incredibly precisely, mid gallop, while sliding down on one side of the horse for cover. Insane.
The speed that a horse-only army provided the Mongols was terrifying for China (and the world) – especially as they mostly relied on infantry troops.
Hence the solution of the wall – it wasn’t meant for the Mongols, it was meant to keep the horses out. Try getting a horse to climb up a steep hill, and then have to build an elaborate contraption to get them up the wall and down it again. By the time the Mongols were able to do that, China had time to bring in its massive infantry troops and stop them.
Except for this horse. This horse had no problems with the Great Wall of China.