Cities are places where a certain energised crowding of people takes place.
Cities come in clusters.
Cities are places that have some physical circumscription, whether material or symbolic, to separate those who belong in the urban order from those who do not. Cities are places favoured by a source of income. Cities are places that must rely on written records. It is through writing that they will tally their goods, put down the laws that will govern the community and establish title to property. Cities are places that are intimately engaged with their countryside, that have a territory that feeds them and which they protect and provide services for.