Any real or fictional city can be claimed, as long as it is realistic. For example, if one were to claim Hogwarts in the Harry Potter movie series as a resort of some sort, it could be done. Members can claim multiple cities at once, as well as entire regions with cities in them. In the latter case, a too large claim (e.g. claiming all of Italy) will not be added to the list. Claiming cities from an entire country subdivision (e.g. all cities of Maine) is tolerated but must be kept to a minimum.
Claimed cities are mostly used for picture material and can and should be altered from their original form (e.g. names, history, etc.) to avoid parallelism. A member can also claim only small parts of a city (e.g. a landmark). Keep in mind that if you claim a real city, the culture associated with that city will not de facto come with it. Population sizes may be altered only if it does not exceed a 15% increase. You may decrease population size as much as you want (as long as it's realistic).