Qiánmén, aka Front Gate, actually consists of a pair of gates: the northernmost is the 40m-high Zhèngyáng Gate, which dates from the Ming dynasty and which was the largest of the nine gates of the Inner City Wall separating the inner, or Tartar (Manchu) city, from the outer, or Chinese city. With the disappearance of the city walls, the gate sits out of context, but it can be climbed for decent views of the square and of the Arrow Tower, immediately to the south.
Partially destroyed in the Boxer Rebellion around 1900, the gate was once flanked by two temples that have since vanished. On the second of three exhibition floors is a fascinating timeline charting the history and demise of Běijīng's city walls and gates, including historical photographs showing the area as it was at the beginning of the last century. Explanatory captions are in English as well as Chinese.
The second gate, the Zhèngyáng Gate Arrow Tower, directly south, can’t be climbed. It also dates from the Ming dynasty and was originally connected to Zhèngyáng Gate by a semicircular enceinte (demolished last century). The site was extensively reconstructed in 1914 according to the designs of a German architect.