Qianmen, aka 'Front Gate', consists of a pair of gate-like structures: the northernmost is the 40m-high Zhengyang Gate, which dates from the Ming dynasty and was the largest of the nine gates of the Inner City Wall. The similarly proportioned building to the south is the gate's forward-defensive arrow tower. This would have been joined to Zhengyang Gate by a semicircular enceinte (enclosing wall), which was demolished last century.
Zhengyang Gate has a museum on three of its floors, and offers good views over Tian'anmen Sq and the arrow tower.
