<P> Town siting has varied through history according to natural, technological, economic, and military contexts . Access to water has long been a major factor in city placement and growth, and despite exceptions enabled by the advent of rail transport in the nineteenth century, through the present most of the world's urban population lives near the coast or on a river . </P> <P> Urban areas as a rule cannot produce their own food and therefore must develop some relationship with a hinterland which sustains them . Only in special cases such as mining towns which play a vital role in long - distance trade, are cities disconnected from the countryside which feeds them . Thus, centrality within a productive region influences siting, as economic forces would in theory favor the creation of market places in optimal mutually reachable locations . </P> <P> The vast majority of cities have a central area containing buildings with special economic, political, and religious significance . Archaeologists refer to this area by the Greek term temenos or if fortified as a citadel . These spaces historically reflect and amplify the city's centrality and importance to its wider sphere of influence . Today cities have a city center or downtown, sometimes coincident with a central business district . </P> <P> Cities typically have public spaces where anyone can go . These include privately owned spaces open to the public as well as forms of public land such as public domain and the commons . Western philosophy since the time of the Greek agora has considered physical public space as the substrate of the symbolic public sphere . Public art adorns (or disfigures) public spaces . Parks and other natural sites within cities provide residents with relief from the hardness and regularity of typical built environments . </P>

What is the main part of a city