A Watchtower on the Wall of the Forbidden City
Old Beijing was consisted of four walled sections. The innermost of these sections was the Forbiffden City, which was the palace where the emperor resided and reigned. Surrounding the Forbidden City was a larger section called the Imperial City. Outer still was another city, the wall of which seperated Beijing the city from the surrounding open fields. Even though this last wall was the outmost of Beijing, the area it enclosed was called Inner City. This was due to the fact that, once upon a time, there was a plan to built yet another walled city around the existing city. In fact, a section of that planned expansion was built, only to the south of the "Inner City," which came to be known as the Outer City. The Outer City, more accurally the Southern City, became the place where commoners lived - farmers who supplied the Imperial City with grain and vegetables.
The Forbidden City and the Imperial City were all rectangle in shape. The outmost city wall of the city measures 24 km in length. The wall of the innermost Forbbiden City is 7.3 km in length, 13 m in height and 16 m in thickness on top. The city has four gates, one each on the four sides of the rectangle. A moat, 52 m in width, goes around the Forbidden City, which, along with the wall and a series of towers on the wall, formed the defense system for the imperial palace.