So some progress was made today--more
cheating with the procgen guiding the generator along to a coherent map structure. Got it placing "special rooms" (well, one, anyway) and generating the surface area, which looks much more like a standard platformer, with little to no issue (after what feels like several issues upon several getting it to work correctly.)
Today's hard lesson, and one I really should've learned by now: If you think your generator will need 100 rules to work correctly, rest assured it will in fact need
200.
Still going to create some more variety in the surface, and I'm way behind on implementing any combat. And also on the realization that ultimately, *not* going single-screen might have given us more flexibility for this project. Probably too late to change my mind on that one, though, and the progress today getting the surface to generate correctly will almost certainly help with making some other items happen, namely, biome transitions, especially to the water area. With the necessary changes, at least.
Also, and this is relatively minor: I moved the horizontal exits up and made them all a little taller. Map traversal was feeling kind of slow and head-bump-tastic and I think that this will help facilitate smoother room designs that are meant to be crossed more quickly.
If I were doing this again, I'd probably look more into slopes than I did, and quite possibly would be using 16x16 tiles with no half-rooms (or with scrolling.)