Very cool place. It is about 50 metres from the Jianguomen station, Exit C.
There is an outside garden with a few observatory pieces. Also, a courtyard with some other pieces and some museum displays.
In the tower, which is actually part of the old City Wall, are many other old observatory pieces. There is also a very good view of this part of city.
Depending on your interests, it’s a smaller place, so you’ll need 45-60 minutes.