Fantasy population centers, motives, and history

[a2area]

in terms of population centers even in a relatively young civilization, climate would be one of the more important factors. That in combination with geographic location and then having a primary population center appropriate to each nation (of sufficient age) or region and its agricultural or economic support (for that population) potential.

If you look at the earth, almost all of our population centers are relatively recent developments.. with the exception of maybe eastern China which has been quite densely populated for a long time compared to the rest of the earth. In anything but the modern age great cities have risen and fallen at the whim of monarchs / military leaders or nature. Perhaps nowadays we have the means to combat the elements to some extent.. but if you take away the reasons for that city being there.. eventually it will wither. Many of the most opulent cities of the near east (for example) have suffered from repeated conquests and never regained their former glory (Antioch).

So basically.. not all of your population centers have to have a very long history and man-made phenomenon like agriculturally induced desertification can play a huge role in herding populations into a relatively small region (Nile Valley). Or colonization can build huge industrial centers that burn bright and die in a cosmic instant (Detroit).
I probably rambled but i hope i stated something helpful?