Thanks guys.
@Virtual Nomad - awesome. I didn't expect anybody to go as far as to "mod" the game, but you're absolutely right.
I changed the code so that the tiles themselves are sprites as well instead of colored boxes, so they can be customized too:
Exiting the game should work with ESC or ALT-F4, as usual - and for me it does. If that is for some reason not the case for you, maybe I should implement a temporary ingame menu.
The mix of bmp and png is due to transparency issues. With bmp's I can use 0xFF00FF as transparent color and thus make sprites partly transparent in paint. However, when using an actual alpha-channel (which I do only for a few sprites so far), png is the better format. Note however, that you can change the sprite file for a unit and for a tower inside the respective .txt-file.
Quote: "enemy path preview prior to placement. i understand that i can add a block and then sell it back to check new paths"
Good idea, I will work on that.
Quote: "i think the mix of pre-determined paths + full AI pathing apparent on the one level(?) with waypoints is great."
Which level do you mean? The spiral one where you have to block certain paths with a few single, well-placed towers?
Quote: "if you leave as much customization available as possible (ie, the replacable graphics, plus sound effects, text dialogue, etc, if/when those are added), players can build their own campaigns with their own themes, premises, etc, to share with others = massive replayability and the "most bang for your (programming) buck"."
Indeed, that is the plan.

The editor is, by the way, not that great. I really only use it for basic map architecture, because that is an irritating task to do directly in a text file. The unit definitions and some other map-depending details are done in the map's text-file afterwards. A strange work flow, but it was the most comfortable one for me so far, even though that's not too modder-friendly (so I might be going to change that as well at some point in time, but not too soon probably).
@Pincho Paxton:
That is certainly one of the weirdest concepts for a game I've heared in quite a while, but also unique and kind of cool.
I will work on the gameplay and the important aspects of the presentation first, but given that the graphics are mostly replacable, such a scenario (even if only for one campaign, not necessarily for the whole game) actually seems realistic.
Thanks for your input once again everybody, it really helps.