Infinite money.
Build your tower to the skies and make money with help of your citizens. There are 2 types of floors in the game: residential (where your citizens live) and commercial (where they work at). When a residential floor is built, you can lift up a new tenant there. Then you may employ him at a commercial floor where he'll be able to produce various goods. When production is over, just hit the Start Sales button to begin earning money!