This should give a better idea of the style I imagined. Since I've nearly finished with the textures I'll complete that job before I do any symbols.

The brownish-reddish grass texture marks a mountain chain that currently has no mountains.

It's only a low res screen shot. Here's a closer look.

I also use the mouse wheel a lot, even though it's actually slower than the zoom tools.

However, I was trying your map with the mouse wheel and still couldn't crash it.

I'm hoping that one of the others will try it and be able to reproduce the crash, but since it's New Year's Day it might be a while before anyone else tries.

You have to choose whether you want to group buildings according to where they are, or what they are. My preference is for where, rather than what, since it's easier to avoid putting things on top of one another if you have all the buildings in the block you are working on visible.

The two level mapping is probably the best idea.

You can (if you don't find it too awkward constantly hiding and showing different layers) do largish cities in one map by putting each district on it's own layer and only having the layers you are working on visible.