Mouse - I like you're idea of having a time limit for the main map, but no time limit on the extra layers that's a good idea.

J.Edward - That map looks lovely, but I think the rivers should stay above ground draining into those big open sink holes, with water falls going into them. It might be neat to even have buildings or tunnel entrances scaling the sides of the cliffs down the sink holes, if that makes any sense. And the citizens could even build big water wheels there to get power for the city... Just a thought.

And a question - what size will we be able to work at in pixels or inches, will there be a limit or will the individual maps be scaled down to fit the big picture after?