-snip-
Neat. I'll see if I can make a map.
As for the apocalypse...hm...maybe some kind of self-replicating magical thingy? A ven Neuman golem, or mitosis-ing spirit?
We should also consider how the druidic totems protect from the apocalypse, and what the side effects would be. If they were meant to protect from the casual waste of civilization, they might well attempt to reverse it, meaning that artificial structures would need to be constantly replaced and cobblestone roads would be basically impossible in all but the wealthiest areas.
Oh, and I had an idea about Oasis. It has a bunch of druidic totems, but those were presumably erected before the city was built. I'm imagining something like the Aztec city of Tenochtitlan, which was conquered by Spain and then slowly turned into a Western city (I think it's Mexico City now?). Oasis could have once been the center of a local nation or organization of druids, which was then annexed by the local nation/nobles/church/whatever and turned into a proper city, albeit with a number of druids (who--along with the magic of the totems--stopped them from being torn down). This city became a popular tourist destination and a center of trade, and then the last bastion of civilization. (That we know of.)
I've no ideas on the apocalypse yet, but I'd say it would more likely come about from organized warfare than a mad wizard cackling in a tower. Maybe even a Mutually Assured Destruction sort of deal.
I the Mexico city idea. Maybe there's not many Druids left these days, but they (and their ruins) could certainly be used as adventurer hooks. Or they could be 'villians'-seeking to take back their land from the many, many invaders. Ideally, if they are almost the sole source of knowledge about the totems they might be able to wield significant power.
And my original idea was that the totems reinforce against drastic climate change in a specific area of effect-they create a 'safe zone' in the center that is always more or less stable. Essentially, they balance out the elements in a given place, and serve to balance each others effects as well. The massive 'western' city that came after was not their original intention-they just wanted to make a paradise that would survive the worst of the ravages civilization...but probably never expected that civilization would be able to ironically conquer the very area they sought to protect. Of course, the totems themselves might not have been fully activated until they were needed.
I would expect the central areas would be, at the start anyway, pretty much stable. Around the totems would be strong elemental biomes of varying sorts-my excuse to pretty much allow any terrain or species in an 'urban' game-and, if existing totems are damaged, destroyed or changed the remaining ones would begin to expand their own power. Everything would eventually go out of whack.