urbs

Urbs, urbis is third declension feminine and an i-stem, which shows up in two places: the genitive plural is urbium rather than urbum, and the accusative plural can appear as urbis alongside the regular urbes. That alternative is a genuine reading hazard, because urbis is also the genitive singular, and only the sentence around it will tell you which you have.

Written with a capital and no further identification, Urbs means Rome. There is no other city. Ab urbe condita, "from the founding of the City", is the Roman dating formula and the title Livy's history goes by, and urbi et orbi, "to the City and to the world", is still the form of the papal blessing.

Keep urbs apart from civitas. Urbs is the built place, the walls and the streets. Civitas is the community of citizens, which can exist without a wall around it and is the word you want for a state or a tribe. Oppidum is a smaller walled town. English took urban, urbane, suburb and conurbation.