1. Vertical city, the size of a small village on the surface. Extends via a wide shaft an unspecified distance into the earth. Eactly how far is unclear because the city’s surface dwellers are unable to descend beyond the point that the heat and pressure become too much to bear for them; those living further down are able to descend further, but not to climb to the surface, which is airless and cold to them; and there are several such transitions before presumably the bottom of the city is reached. Rumours pass regularly back and forth, as do heat-suited explorers who never come back. Meanwhile, there are those who say that an identical city exists on the other side of the world….
2. A city shared by daytime workers, who use the sleeping areas at night, and nighttime workers, who use the sleeping areas by day. Each group is entirely sure that the city becomes a nest of villainry whilst they sleep, best avoided at all costs. The different inhabitants of the sleeping areas never meet, but will often write each other notes, for example explaining if something has broken or requesting that some foodstuff be left alone. However, they all know that they cannot trust their home’s other inhabitants, who are probably liars.
3. A city that has come to terms with the dead coming back to life; the event where they began to rise having occurred some time ago, and having proved to be surprisingly containable and not the end of the world after all. Now there are areas of the city where the living live, and areas of the city inhabited by the dead, and even some limited commerce between the two. Space is perpetually a problem; although the dead towns are small at the moment, their population can only go up. The more disintegrated dead can fit themselves to the spaces available, although sometimes they block important vents or pipes. The less disintegrated dead argue that they should be able to set up mausoleums in the green belt, but nobody has updated planning laws since the event. It is a bureaucratic mess.
4. A shining city in the mountains, viewable from afar, notable for the symmetry of its inhabitants’ motions and for the intricate tesselations of its towers. In reality it is just a single large building set amid a vast field of mirrored rocks. The building is a monument to whatever happened to coat the rocks with metal, and the small number of inhabitants there tend to the monument and run a small farm. Sometimes birds descend on the city in vast fractal flocks, and then the people in the valley below shut their doors.
5. A city on a vast stone slab beside a lake, inhabited by frog people. If everyone runs to one end of the city the slab tips over, flipping its overwater and underwater parts. This is done ceremonially twice a year; but may also occasionally happen by accident, for example when there is a particularly popular concert on one side of the city.
6. A city existing mainly as data. It stands, when it stands, on a wide plain on a desolate moon. Every few tens of years it is destroyed in some way or other. Then the fabricating machines come out from under the surface, and those of the inhabitants who remain discuss which of the city’s backups to restore from. There is a particularly acclaimed square with a fountain at its centre which has made it into almost all of the final versions of the city. Other than that it is usually a mix of parts that the city has had before, depending on the tastes of who is putting it together and the political climate of the time. One can never be sure if a particular suburb will be there in the physical city or not; but if not, it remains in the data, ready to come back some day.