The Great Wall has three major components: passes, signal towers (beacons), and walls. The design of each of these features has been determined by strategic importance.

 

 

also known as beacons, beacon terraces, smoke mounds (because of the various substances, including wolf dung, that were burnt), mounds, or kiosks.

Communication between the army units along the length of the Great Wall was of prime importance. Not only was it necessary to be able to summon reinforcements in the event of an attack but it was necessary to be able to warn other garrisons of the movements of enemy forces. Making full advantage of hill tops and other high points along the wall for their location signal towers were built. These would vary from being complex structures of more than one storey in height to simple beacons. During daylight hours smoke signals were widely used and to create varying colors and density of smoke many different materials were used. Possibly one of the most bizarre of these was wolf dung! At night lanterns and beacon fires were used. Other means of signalling included the use of flags, clappers, drums and bells. The invention of gunpowder also proved to be an asset to the signal system as the firing of cannon made a sound that could carry over long distances. Codes were devised that included a combination of cannon fire and smoke signals as well as other devices. For example, during the Ming Dynasty a single column of smoke plus a single gun shot would indicate the approach of a hundred enemy soldiers. Multiple columns of smoke combined with an appropriate number of gun shots would give an indication of the size of the invading army.

The complex signal towers would provide accommodation for soldiers as well as storage for ordinance and even stabling for livestock and horses.