direction coming from forbidden city

To your demand, subway may be the best choice. You can take subway line 1 at Tiananmen East or Tiananmen West Station to Fuxingmen MTR Station, where you can transfer to subway line 2 and get off at Xizhimen Station. Then transfer to subway line 4 to Beigongmen Station, which is close to your destination.

This garden is very popular so that many people go there every day. Though, if you happen go there during Chinese holidays (especially May Day Holiday and National Day Holiday), you will find it is crowded for a whole day. If not, it will not be that crowded.
Take easy!