Golarion: 300 ARA
Ezanzalamporfaluzorion, or more commonly Elza, is one of the most populous and well known cities of Golarion. Elza has a population of about 1 million, roughly 90% of which are gnomes. It is doubly unique, firstly in that it is the only primarily gnomish city in Golarion, and secondly, in that it is also the only flying city in Golarion. It is also the youngest major city of Golarion.
Around 200 ARA, there was great interest in the development of flying buildings (primarily just interest in the possibility, there is no significant practical application). The Imperial Academy of Transmutation cast a series of extremely powerful fly spells which must be constantly maintained and directed to cause their Academy to be able to fly. Then, the Gnomish Mage-Engineer Julian Fritx-Fynn-Fadacious Fudwick developed the trick of strategically placed Reverse Gravity fields to counter gravity throughout the structure that was to fly. After some initial trials, a city was commissioned that was a single piece, sturdy enough to sustain the stresses of the bi-directional gravity, designed with some buildings inverted, and large enough to support the anticipated population (Elza is now at roughly 50% capacity).
Several large masses can be moved into and out of Reverse Gravity zones to maintain altitude. Remotely controlled Gusts of Wind on the edges of the city propel the city laterally. Elza flies around Golarion, periodically stopping near major cities, and has on several occasions docked to the Imperial Palace.
Roughly half of the city is constantly under the effect of Reverse Gravity spell, but these areas are spread throughout the city, and the transitions can be very sudden for those not familiar with Elza. The inhabitants are likely to be seen summersaulting at these borders. There are several places were it is possible to fall indefinitely, oscillating between gravitational directions. The city is brightly colored and extremely shiny, and is generally one of the places in Golarion most like the First World.
As the only Gnomish city in Golarion, Elza is the center of Gnomish engineering, which blends magic, mechanics, and chaos into a stunning array of contraptions. Elza, unsurprisingly, has a particularly high concentration of flying machines of every description, from dirigibles through alchemical dragons, through experimental fixed wing aircraft with magical propulsion.
While it is possible to reach the city by flight (magical or mechanical), or during its rare surface contacts, most traffic through Elza occurs via its numerous Interail stations.