Beckon Towers is one of the most vital components of the system, an effective mean of military information transmission. Though already widely used long before, Great Wall took even better advantage of it and perfected it into the best for message passing in ancient times. Specially, it used smoke signals at daytime, and fire at night, for fire was hardly visible in bright sunlight while easily noticed from afar in dark. It is a fairly scientific and fast method. The number of invaders was reported by special number of fires or smokes. In Ming Dynasty, explosion sounds were added to reinforce the effect. In a time of no telephone or radio, this relay of information was practically effective and fast.
Beckon towers were usually placed at high and steep peaks or the turning points, so as to keep in sight of the neighbor towers for continuity. Apart from military functions, they also provided protection, boarding, and supply for the passing envoys.
