Which areas in a world are urban civilizations (that is, civilizations characterized by large population centers or cities) more likely to develop?
I submit that urban civilizations typically begin in small, agriculturally productive regions surrounded by large and less productive land. Fertile river valleys bordered by mountains or deserts are the most common examples of this. To have a city, you need a lot of people concentrated in a relatively small area, and the type of area I have described allows for that. If the agriculturally productive land spreads out too far and wide, then instead of cities you will have lots of small rural villages dotting the landscape.
Alternatively, cities may develop alongside trade routes, although such settlements require at least two existing civilizations somehow geographically separated.
Can anyone else name more conditions conducive to urbanization?