I actually built the entire map in the editor first and then sliced it up and added detail/colour in paint.net. The entire city world was drawn there in one piece. Here's a screenshot of the world map as seen in the editor (before i placed scenery tiles):
The dotted path lines you see are removed when the world map is loaded and are placed so that you can always access any level.
The map actually has a perspective error which I missed in editing. Can you spot it? In-game it's covered by scenery tiles.