This is the forward-defensive structure of Zhengyang Gate, which sits behind it. The imposing building is a jiànlóu (arrow tower), which would have been joined to the gate by a moon-shaped enceinte (enclosing wall), demolished last century along with the city walls. All told, it would have formed an awe-inspiring defensive barrier. The arrow tower cannot be climbed.
Like the Zhengyang Gate, it was burned down during the Boxer Rebellion, and was subsequently rebuilt. The site was extensively reconstructed in 1914 according to the designs of a German architect.
