The Cities are the place of the governing body of the Nox. They are also a main meeting place. They were home to the main population of the planet, but some of them lived in small groups all over the planet.
The cities can float in mid air while holding their position relative to the ground. The cities, like all Nox technology, could turn invisible even though they were very large, it is unclear whether this is some form of cloaking device, or another use of the abilities of the Nox themselves, in their ability to cloak creatures and devices, such as the Tollan Ion cannon, which Lya hid during a Goa'uld Attack. The cities are similar to Atlantis in some aspects, but the Nox, unlike the Ancients who built Atlantis, didn't make the Stargate a key place inside the city and instead left it down on the planet surface. (SG1: "The Nox")
The appearance of the Nox city was updated in the clip show episode Out of Mind. No official reason was given for the changing of the CGI, but there is a clear difference between the original city and the updated. The updated city seems much more technological while the original has natural and even mystical attributes, such as a forest and roots growing on it. While there is no known reason for the update, since the change was made, for whatever reason, it can be assumed that the more recent depiction of the floating city, is the canon one.