Thank you for playing and for providing so much feedback. Much appreciated :)
I completely agree the NES would probably flicker like crazy from all the huge sprites and depth sorting going on!
I'm really glad you liked the bat cave level, that was one of my favourites too and I had planned a second cave-themed level but it got cut towards the end as the enemies I'd designed for it weren't properly fleshed out.
Thank you for your comments on the difficulty too! Really helpful :)