I believe once every week, there is an city update. In this update the system calculates how many citizens you have in your city. To advance is to make those requirements. However, Rank 4 and 5 are very difficult to obtain since there is a planetary cap for Player Cities. You lose progess if by the next update you fall beneath the requirements and some structures will be automatically destroyed.
Following Rank and Requirements:
Citizens: Rank
5-9: Outpost
10-14: Village
15-29: Township
30-39: City
40+: Metropolis
Edited, May 27th 2007 7:00am by AayliahOfRemora
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