The mayor states that the Chinese built a wall that kept Mongolians out for centuries. This is not true; the Great Wall would have been far too large a structure on which to post and supply guards. The Great Wall didn't keep people out; it kept things in. Raiders could scale the wall, enter China, and plunder. However, porting all of that loot to the other side of the wall would be a considerable effort and take enough time that whatever served as law enforcement in the area could probably catch up with the raiders.
Response: What you said is wrong; the Great Wall of China was actually built to protect the various dynasties from raids from the neighboring tribes. The structure may have been too big to post and supply guards but that's why they had stations where there were guards to start a fire to alert soldiers if an invasion ever occurred. You also contradicted yourself, if scaling the wall from the outside was easy, then scaling the wall from the inside would be no different.