The wall and the soldiers who guarded it were part of an elaborate defense system across northern China. Nearly a million soldiers patrolled the Great Wall, but they were spread thinly across thousands of miles. The Mongol warriors were outnumbered, but they had an advantage. Thanks to their swift horses, they flowed like water across the steppe. They could assemble anywhere, at any time to launch an attack and then disappear just as quickly back into the steppe. To defend against their fast-moving enemy the Chinese used an ingenious system of communication to gather soldiers together for battle.
Stone platforms, called signal towers, were built on high ground near the wall. When Mongol horsemen were spotted, a smoky fire was built on top of the nearest signal tower. The smoke was visible for miles, and when guards at the next tower saw it they built their own fire, passing the signal along. Sometimes loud cannon shots accompanied the plumes of smoke. The number of smoke plumes and cannon shots was a code indicating how many enemy riders were approaching.