To the north and west of Ancient China were two of the world's largest deserts: the Gobi Desert and the Taklamakan Desert. These deserts provided borders that kept the Chinese isolated from the rest of the world. The Mongols, however, lived in the Gobi Desert and were constantly raiding cities of northern China. This is why the Great Wall of China was built to protect the Chinese from these northern invaders. Because the Mongols were so powerful with a massive army, China couldnt expand any farther than the Great Wall of China. This gave the Mongols an advantage in forcing China and its empires under their control.
I hope this helps! Please correct me if I'm wrong :)
