The population of the sky is rather thin. Only few men live in the sky, and also few animals and plants do; the birds are sometimes said to inhabit the sky, but this is more or less just a rhetorical manner of speaking: in fact, the birds are usually living in trees or bushes and visit the sky just temporarily. So it is no surprise that there is just one major city in the sky: just one city at all, to be exact. This city is called Laputa and was first descriped by Lukian.

Here you can see a picture by M. C. Escher. The mechanism of levitation was first guessed to be based on magnetism (see, for example, Swift, 1726) or rocketry (for example Adams, 1982), but in fact it is based on a principle of the superstring theory in quantum physics far too complicated to explain it here.
The name of the city derivates etymologically from "large reputation", that means "the very honourable city" (and not from the spanish "la puta", as some scientists asumed). The government is a representantative anarchy, the economy is based on a liberal stalinism. There are approximatly 120,000 persons living constantly in Laputa. About 17,000 of those citicens are of noble birth and have wings; the rest of the population is forced to visit the ground with ropes or helicopters. It is forbidden by law for the nobles to marry one of the other citicens or one of the people living on the face of earth.
Legends say that Laputa was originally invented as a weapon for warfare, but this is certainly untrue, since the city is fragil and unable to take part in combat. In fact, it was invented to allow the king to visit all parts of his contry together with his court without discomfort, since the king was expected to travel trough his land all the time. The only time Laputa was used in a war lead immediatly to its destruction. 3,000 years after this event, Laputa was rebuilt by the Disney company as an amusement park.

There is a sad bittersweet story about a noble girl from Laputa falling in love with a man from the earth in over 7,000 verses by a gifted poet.