If your city reached a certain amount of people, Dr. Simtown will reward you with a building which may help your city.

Building	How to unlock
Center for the Arts	Reach a population of 100,000
Court House	Reach a population of 25,000
Mayor's Manor	Reach a population of 5,000
Medical Research Lab	Have a population of 80,000 when you reach the year 1999
Museum	Reach a population of 3,000
Post Office	Reach a population of 100