It is hard to balance population. Just as in Sim City unemployment can be a big problem because citizens become unhappy. Just enough population needs to be attracted to support structures. Or of course you have to build structures to employ the population. Areas need to be attractive to attract the rich and famous to a city.
Structures have a positive or negative effect on an areas desirability.
Funding is a large aspect of the game. Importing and exporting often are needed to support city funds. Tax collection agencies can be set up. With that of course the tax rate must be set. On the other hand you may recieve gifts from other cities if you have a high rating.