Beacon sites were high platforms with facilities to create warning signals - smoke by day and fire by night. Under the platforms were soldiers' sleeping quarters, stables and warehouses. These were built with stones or packed earth, or a combination of bricks and stones.
Beacon platforms took four forms: close to the Great Wall, outside the Great Wall, linked with a royal prefecture or county, or linked to a neighboring prefecture, county, mountain pass or fortress town. The location of these beacon platforms was specific; all were in view of two other platforms. Protective walls surrounded the platforms in border areas. The number of smoke columns or fires depended on the strength of the enemy: 50-500 warranted one column, 500-3,000 two columns and so on. Each tower had six furnaces plastered with lime. There were also signals in the morning and evening to indicate that all was well; if the signal never came, the tower was assumed to have fallen to the enemy.
A complete beacon system rose under the Tang, a dynasty which did not build or add to the wall in any significant way. Their system, which incorporated previous systems, included alarm raising and changing guards. Wolf dung was often used for making smoke signals which stayed in the air for longer than smoke created from other substances. The wall and signaling was improved under the Ming. In 1466, regulations introduced cannon fire into the signal system. The number of shots specified the strength of the enemy.
