Thanks Cindy for your prompt reply, but according to the information provided on this page,

You are welcome, Gardenia. I checked again and there are related information about the sightseeing bus. It is from the South Gate. The city bus station in front of your hotel is called Sa Jin Qiao. You can take bus 12 and get off at the South Gate. Find the sightseeing bus at the square outside of the city gate. It runs only along the City Wall, leaving every hour on the hour from 9am-6pm. With that bus ticket, you can get off at any stop and have a short visit and then get on another bus to the next stop. There is also battery bus on the wall, 50RMB for a circle.