There's a good game in here, but it felt like the Game Boy wasn't a good choice for this sort of thing.

I can appreciate the ambition, and the first three levels felt fine, but there's some pretty noticeable frame rate issues, which brings down the enjoyment you get. Platforming becomes more tedious, the combat in slow motion doesn't feel good, and the incredibly minimalistic presentation removes a big part of what makes the retro Castlevania games so special.

And there's stage 4. It felt a bit too long and accentuated another problem with the game, which was its poorly placed checkpoints.

Had it been designed for another system, this could've been quite good.
In its current state, it's something that can be safely skipped.