Cities generally have complex systems for sanitation, utilities, land usage, housing, and transportation. The concentration of development greatly facilitates interaction between people and businesses, sometimes benefiting both parties in the process, but it also presents challenges to managing urban growth.
In ancient Greece, beginning in the early 1st millennium BC, there emerged independent city-states that evolved for the first time the notion of citizenship, becoming in the process the archetype of the free city, the polis.