A world map - DL to get the full effect (and even then, it's chopped down to 25% of its original size - this thing is HUGE). I focus on Arna, Calpolli, and Ayotl when I'm writing, usually. I'll go to those other places eventually!
The original, basic idea, torn down to its bare roots was "secret history of the world" - Aesca was supposed to actually be Earth 90 million years ago, and even though that's changed, I've still retained the map, because it's awesome.
In my head, I'm all like NOT DETAILED ENOUGH. MOOOORE.