Also interesting is the fact that most of the game was written in C. I was under the impression that anything of this relative complexity would've required more hand-tuned assembly.
Most of the heavy lifting, like the special effects, is done in asm as one might suspect. For example, it does away with GBDK's standard interrupt multiplexer in favor of its own asm code. It's not really surprising that a game that requires little realtime input has delegated "household tasks" to c, which then controls the engine. In fact, doing so may be good for your sanity. It's the principle of optimizing the 10% of the code that does 90% of the work.
Nice seeing that this game was finally released. For every year that passed, I had less and less hope that it would happen.