Built in 1420 to protect the imperial palace perimeter, the four corner watchtowers (Jiaolou) served a critical military defense function alongside the palace walls and moat.

Architecturally, these structures are masterpieces of ancient Chinese timber carpentry. The intricate design features 9 beams, 18 pillars, and 72 roof ridges that support a three-layered overhanging roof of imperial yellow tiles. Today, they remain iconic landmarks within central Beijing's historic core, celebrated for their sharp reflections across the surrounding moat.