Nice designs on the bosses, very unique and (from the ones I've seen so far) diverse from one another.
I genuinely got excited when I unlocked the HP upgrade after Chapter 1 :)
Unfortunately I got stuck in Chapter 2. The spikes where tough to navigate since I didn't have a visual or auditory cue of when they would pop up. I don't know if you're planning to do any version updates or not, but if so I'd suggest trying a mechanic where the spikes are always visible in their placement but are only one or two pixels tall. They player still won't know when they pop up, but they would at least know where they potential danger is. Might be too easy, I don't know.
Lastly, I seemed to have trouble with the castlevania-esq power ups (which I'm a big fan of by the way). In some cases I could throw unlimited crosses and other times I was able to throw little to none despite just having grabbed the power up. Is there an ammunition mechanic that I missed?
Nice job with the game; it's clear that you put some dedication into its creation. And while I haven't tried any 1-bit development myself, I do enjoy seeing how others make use of the art style's limitations.
PS - I caught the Fix My Heart reference as well, nice touch ;)