From Dongsi we took a bus going to Jingshan Park, make sure to study the routes before taking a bus so you'll not get lost.
The focal point is the artificial hill Jingshan, literally "Prospect Hill". [Reference]
It took us more that 20 minutes to reach the pavilion on its peak.
How to get to Jingshan Park (see map below)
Address: Jing Shan Gong Yuan 44 Jing Shan Xi Jie, Xicheng Qu, Beijing Shi, China