Emperor Wudi (from the Han Dynasty) invented this particular kind of towers. These alarm towers were located on the ridges, every twenty to forty kilometres, on either side of the wall, according to the best view point. They had a double role: to advise the army as quickly as possible, but also to spread the information from one end of China to the other. Depending on the ambient brightness, the lookout men ignited wood to produce smoke or lit up a clear glowing fire: the message was then visible from the Great Wall and to two neighbouring alarm towers. A single column of smoke meant on: an attack by an army of less than five hundred men; two columns: less than three thousand men, etc… The news was thus more efficiently conveyed than on horseback. Some old towers were found with heaps of straw or reeds, ready to be set on fire at any warning.   + de photos ...