This is nice! I when I was a kid I had a computer that ran on Z80 processor, I used to program things in Basic on it, but assembly was way out of reach (back then the only way to learn something was to take a course or buy a book, and sure assembly was out of the question). Your game took me back to those old days (much better days compared to my present). Enough talking about me, let me talk about the game: I like how much effort and love you put in here, your interpretation of the theme is very good, and although you restricted yourself with the hardware you used, you were able to use what was available in a good way. I found myself dying a lot, and it took me sometime (and some reading) to understand what was going on, this needs some tweaking, I think.
All in all, this is a solid entry and I really like it! May be you can make some series on how to make a GB game :)