A remote archaeological expedition in the Hexi Corridor has unearthed the remains of a Han Dynasty watchtower that served as a specialized hub for 'Frontier Signal-Mirror Calibration.' Excavations inside the tower's sealed basement revealed a collection of carbonized bamboo slips containing the earliest known codes for heliograph encryption used to relay military intelligence along the Great Wall.
The slips detail the precise angles and flash sequences required to transmit complex messages across long distances using polished bronze mirrors. This discovery sheds light on the sophisticated optical communication network of the Han Empire, demonstrating how frontier garrisons used pre-arranged encrypted signals to alert the capital of nomadic incursions with remarkable speed and security.