China's rulers knew that a defence system to protect the country from invaders needed to be substantial. But how substantial would it need to be? A lengthy wall was constructed over time, but its length and its height was not the only strategy employed in defending the motherland.
Due to its length, the Great Wall of China could not have men along every inch of its ramparts at once watching out for the Mongols, Turks, Tungnu, and Xiongnu. Even with nine Zhen (military districts) covering the 4100 mile length between the First and Last Doors under Heaven, the defenders held true to Sun Tzu’s adage “He who defends everything, defends nothing.” To prioritize their defence, the Chinese created a method to alert garrisons where attacks were in progress. This took the form of a firelight defence system within the watch towers.
The Great Wall is not merely one long wall meant to keep invaders out of China. Rather, it is dotted every so many miles with towers, numbering 25,000 in all. These towers served two purposes. First, soldiers could be posted in the tower to keep watch for anyone approaching the wall. To maximize their line of sight, and therefore, the time they had to warn of potential invasions, the towers were built on hilltops.
Second, should an attack come, the soldiers in the tower could light a signal fire. Each tower guard could see either the smoke in the day or the light in the night and start his own fire (the aforementioned choice to build the towers on hilltops also guaranteed both fire products would be visible). This would continue from one tower to the next until every available troop was alerted and ready to defend the wall. Additional messages were also sent via lanterns mounted on long poles and flags.
Soldiers also patrolled on top of the wall from tower to tower to spot any movement the tower guard might miss. The whole length of the Great Wall collectively garrisoned tens of thousands of peasant soldiers by 102 B.C. before it had even finished construction. Wide passages were constructed along the tops of the walls to expedite the movements of large numbers of troops and couriers. This made defending the wall’s prodigious length both logistically and strategically feasible.
Thus, the Great Wall of China was defended by watchful eyes, a system of signal fires, lights, and flags, and an army of brave, dedicated, and an army of observant men who were quick on their feet.