Thank you, glad to hear you're enjoying it!
I might have to shatter the illusion a bit, but there's actually not very much map detail stored. The world is made up of just 35 rectangle and oval shapes drawn to the screen and copied to map memory. The trick is that the shapes are drawn over 64 cel-like room shapes composed of several hundred randomly-drawn black rectangles. I hit on this after initially trying pure procedural generation, which was too chaotic. Combining random detail with specifically-designed features seems to give most of the effect of storing a lot more information, though you have to work with what's there. It's kinda like looking for faces on a textured ceiling. =)