On the subject of music, I recommend you check out:
This artist allows free use of the tunes available there for RPG Maker and I assume Game Maker. I’ve used quite a few for my own project, since it’s also Castlevania inspired. Some of the songs also have 8-bit renditions. At the very least, I would consider adding them in your own play test sessions to get a better feel for what you want your levels to ultimately sound like.