Over the past two weeks, I've been dissecting
Castlevania II: Belmont's Revenge code and data. At first I was just intending to make some notes for someone else to use in a romhack, but I ended up making a proper
level editor.
It should be possible to make an overhaul hack using this. There's still a little more work to be done to really unlock all the potential, like figuring out the enemy graphics loading routine. (All I really know is that it happens during door transitions.)
Contact me (NaOH#1432 on Discord) if you'd like my notes on the assembly code. I've figured out a decent chunk of bit of bank 0, bank 3, and bank 6. I've also put some notes on
Data Crystal.