I believe it can be done. It wouldn't be a city like we know. Each building would have its own power source. The power would come from wind turbines and solar energy, which would power a cycle fan below the structure. The fan would draw in air, and force the air out through vents facing down, thus propelling the building up, and eventually to a point it which it would float. and below each building would be a parachute-type fabric to give the propelled air something to bounce off of, creating a current for the floating action. These tarps would be deployed once the building is high enough. And if there is not enough air strength to launch the building, you could use a blimp. The blimp could possibly stay attatched to lessen the amount of power the fans would need. Each building would be surrounded by a "yard." The yard could have grass and plants. The walk ways would be made of a very light metal, to reduce weight. Every building would be its own structure. For streets, they too would be their own structure, not connected to buildings permanetly, in case of emergencies. The streets would simply float next to the buildings so citizens could easily walk back and forth. There would, of corse, be no vehicles due to their weight. In case of a building falling, a parachute would be deployed. This project would be very expensive, and some major research would have to done for the fans, too. I believe my theory is a stepping stone for inventors
