No work tonight, which gave me chance to have a good session with Blitz and get the Editor working in a proper window. The main problem with this is really the lack of documentation that comes with Blitz - every command is explained, but seldom few are properly discussed or come with examples. This paves the way for a lot of command clashes and errors, and it's often a nightmare to work out what's going on.
Nevertheless, patience reaped its rewards and after a couple of hours tinkering I had the editor running in a proper window, file loading and saving was sorted (the previous system used a DOS box and was far from graceful), and the mouse scaling issues were rectified.
To finish off, I grabbed the map from Citadel's second level on the C64 emulator and stitched it together in PhotoShop (the reason being, I'll soon be implementing multiple maps and tilesets and will need visibly different data to use). As an afterthought, I also devised a neat way of generating collision maps by allowing the user to store collision data for every tile; that way, their code can either use the collision map as an "edge of road" map, or check for collisions on a per-tile basis.
