Beacon towers (烽火台) lined the Great Wall — smoke by day, fire by night. The infamous tale of King You of Zhou (周幽王烽火戏诸侯) has him lighting them for a smile from Baosi, and when real invaders came no lord believed him.
usage
Figurative sense 'flames of war' is very common: 烽火连天 = 'beacon fires filling the sky,' i.e. war everywhere.
