I don't know whether OP is the author of this code, but if they are, or if anyone is keen to use it as a template, there's a small but important quirk of the Game Boy hardware this doesn't take into account: when the Game Boy's screen is rendering (i.e. almost all the time), the CPU doesn't have access to VRAM.

In this case, I guess this demo works (if it does, I haven't tested it on my flash cart) because the amount of data transferred to VRAM is small and the timing of things happens to line up so that it all gets sent at the right time, but for anything more complicated you can't rely on that.

There's two ways around this: either turn the screen off before copying your tile and map data over (and you can only safely turn the screen off when it's not rendering, too) or set up a system to blast as much data as possible into VRAM as quickly as possible when a frame is finished drawing.

To test if the screen is off, either spin until bits 0 and 1 of $FF41 == 01 or set up a interrupt handler that'll get fired on vblank.

OP, if you did write this code (or if the author reads this, or if anyone else is interested), feel free to hit me up with any questions. My primary side project is a GB game, and I'd love to talk shop.