Great Wall Beacon Tower Picture

A signal system served to communicate military information to the dynastic capital once existed. The beacon tower on the Wall is on mountain tops within sight of the Wall. Signal towers were built upon hill tops along the wall for their visibility. Smoke signals were used in the day and fires were lit at night to communicate with other military outposts along the wall. At the approach of enemy troops, smoke signals gave the alarm from the beacon towers in the daytime and bonfire did this at night. Emergency signals could be relayed to the capital from distant places within a few hours long before the invention of anything like modern communications.