Bongdon, the beacon signals, were used by military to send messages around the fortress to other soldiers. There are a total of five similar chimneys that were used.
During the day, smoke was used to transmit messages. At night, fire was used.
When one was lit, it meant everything was normal and no attack is imminent. Two, an enemy was approaching the national border. Three, the enemy reached the national border. Four, an enemy crossed the national border. If all five were lit, the enemy was attacking.
Similar beacons are located at Namsan Park in Seoul.