Fantasy population centers, motives, and history

[Xyll]

If you are looking for motivation then you could always use a God of Death/Slaughter waxes and wanes in power and every thousand years the god is at his peak and go from their. With the players trying to stop its minions or in reverse trying to save the "savior" who allows the world to return back to its normal state. I think every 500 years is a bit soon as your long lived races could have lived through multiple cataclysms and the recovery rate would not be that fast.

Another theme could be a multi part artifact/item/spell that the players are racing around trying to keep out of the wrong hands.

Your options are endless.
However I think the time frame should be on the milenium scale not the century scale.

[macbeth]

I'm just beginning to design my first world right now (just drew up the continents today) and I definitely plan on making sure I know the "why" and "how" things are where they are and making sure things make sense. I want my players to be able to use magic and have casting characters without too much predjudice against them, but I want magical items to be a bit rare.

To make this make sense, I'm planning on a world where magic users are pretty common, but magical artifacts and tools are not so much so. However, magical items were a bit more common during a past civilization that had fallen, and even more powerful ones were more common in a civilization before that where the ruins are harder to find, and when found, much less often are they unpillaged.

To explain this, I'm imagining something along the lines of what has been said in many of the early responses to this thread, a waxing and waning of the power and knowledge of civilization through some force, whether it be gods, the irresponsibility of powerful civilizations, etc. One path I was thinking of following is along the lines of while the goodly civilizations gain in power on the surface, those of the Underdark do as well and every few millennia become powerful enough to invade and almost totally wipe out the surface dwellers, while also overreaching themselves and being beaten back into submission by those that remain.

It doesn't have to be the same thing every time either. One time it could be that the civilization was conquered and overrun, another it could be that they over reached their bounds with magic and were punished by the gods and had it, and the basis for their advanced civilization, taken away for a few generations (I like that idea, thanks Bladesake). I know one of my catastrophes, probably one of the most recent two, will be a plague that only targeted elves and wiped out 95% of their population. This is how the humans managed to become the dominant race in the world over a race that is much longer lived and much more wise and knowledgeable than they are.

Anyway, I'm rambling. But yeah, every decision I make to place a city or to populate a general area is going to have to make sense. The hard part is trying to figure out what extra factors come into play in a fantasy world. I know I'm going to have to have city states where the population huddles close to the city because of the nearby goblin infested mountains, and that city state won't be able to advance how it would have in the real world.