What's the evidence they presented to support the idea that it didn't stop nomadic invasions? The Mongolian were made of smallish tribes that rode around on hor

What's the evidence they presented to support the idea that it didn't stop nomadic invasions? The Mongolian were made of smallish tribes that rode around on horse. I can't imagine that being a force that easily circumvented the great wall. I know the end of the wall wasn't a very secure area, but that's because obviously there's a lack of wall.