Part of the "Megastructures: The Visual Encyclopedia" book project, on sale now at http://www.neilblevins.com/megastructures/megastructures.htm
A Cloud City is a category of structures that would allow your city to float above the ground unaided by hanging (like a Ceiling City) or pushing from the ground (like a Megatall Skyscraper). The first type would not be that different from modern hot air balloons, although any lifting gas could potentially be used. Another type would be a city that uses flowing air to remain aloft, like perhaps a battery of large helicopter rotors surrounding the structure. Another type would use magnetic levitation to rise above the planet’s surface.