I finished my game! I even added a song, which sounds straight out of Charlie Brown.
It's pretty solid! My wife is playing right now and sounds like she is encountering some oddness with the "good/bad" noises it makes, but what are you gonna do? It's an LD game. It seems to work mostly. I never did implement a check to ensure levels are winnable, but I prevented it from using any of the blatantly impossible rules (like "Tiger must be next to Panda" combined with "Panda can't be next to Tiger"). I only encountered one impossible level ever, and it was because of one of those combos I prevented, so maybe it's fine. One thing I never did do was prevent it from starting in a won state... so that may happen to you! Hasn't happened to me yet.