El Castillo's four sides each have 91 stairs that, when combined together and including the temple platform, equal the 365 days of the solar year. Each of the nine terraces is split into two sections, for a total of 18, representing the number of months in the Maya calendar. The upper section of the stairway was probably used for ceremonial purposes while the lower section may have been occupied by common people. The outer walls of El Castillo are divided into nine horizontal panels, which represent the nine planets of the Solar System. These panels were most likely added after the building of the structure itself because earlier drawings show that there were only six panels. The inclusion of the seventh planet, Uranus, could be an error because it wasn't discovered until 1781 so it would have been impossible to know about it then. However, some scientists believe that Mars might have been meant to represent Uranus because they both have similar sizes and orbits within the Milky Way Galaxy. Chichen Itza was one of the first cities in Mexico to be built up around an urban center rather than on a peninsula or other elevated ground. This type of city layout is now known as a "central-place" theory city because it was designed with trade routes coming in from all directions as well as water access for fishing and trading. The stairs and panels of El Castillo were not part of the original construction of the city but were added later.