Coruscant is a world that's one giant city. There's a couple like that, but Coruscant is the biggest and oldest, and also the capital of the galaxy. It has a population in the trillions, and when someone wants to build something new, they just stack it on top of another building since there hasn't been any open space for tens of thousands of years.

You can tell where the rich and powerful live because they can afford 10 meter ceilings and picture windows. Plus they stay on top, literally.