The Great Wall of China at Badaling

11. Walking to the Guard Tower on the Great Wall

From these towers the Han troops used smoke by day and torches by night to send messages along the length of the wall. They burnt wood, and straw mixed with wolf dung, which produced rich black columns of smoke. Statutes strictly governed the code of these signals. One column of smoke indicated an impending attack by a force of less than 500. Two columns indicated an attack by a force less than 3,000 and four columns indicated an attack by a force of up to 10,000.