After recently rereading Thucydides, GC asks, "Fire-signals of an attack were also raised towards Thebes; but the Plataeans in the town at once displayed a number of others, prepared beforehand for this very purpose, in order to render the enemy’s signals unintelligible." 

I would have imagined that a fire signal was quite simple, fire or no fire. This implies much greater sophistication. 

What is known about this?'

Join us on Patron patreon.com/ancientwarfarepodcast