it is interesting to note that many of the savanna urban centres appear to have grown up at environmental interfaces, between transportation systems. Thus at Timbuktu goods were transferred from camel to canoe and at Kano from camel to donkey. (page 141)
When you’re drawing your map of the world, put cities at the border between two terrain types. That’s where traders change from one type of transportation to another. For instance, horses come in the north gate of the city, and camels go out the south gate.
It’s well known that cities grow up along coasts and rivers. That’s where ship- and raft-based travel meets road travel. But that’s just the most obvious example of this general rule.