Beacon towers were built on both sides of the Great Wall at the commanding points, which were at the top of the mountains or the twists and turns for making warning signals. In ancient times, probably fire and smoke were the most efficient ways for communicating these messages. Whenever the enemy was sighted, fires were lit on the top of the beacon tower at night, and the smoke was made during the daytime.
The smoke was specially made by burning dried wolves’ dung in daytime. (The smoke of the wolves’ dung stayed closely together, went straight upward quickly and dispersed slowly into the air, so it could be easily seen from a far distance) Moreover, the number of the fire and smoke signals could reveal the number of invading enemies. The signal of a single fire or smoke with one shot of artillery fire, warned the invading troops of 100 enemies; two smokes and two shots of gunfire indicated 5,00 enemies; if the number of the coming enemies was around l, 000, there were three smokes and three shots of gunfire. Four smokes and four shots of gunfire indicated over 5, 000 enemies, and 10, 000 enemies with five smokes, etc. In this way, the commanders would not only know where the enemy was approaching, but also the numbers of the enemy. Then they could be well prepared for defense against the enemy.