Thanks! If you'd like to try it the demo is available; it's a complete game of it's own, usually takes just under an hour to 100%:

https://jmkdev.itch.io/depths-of-europa

The Gameboy, like the NES and other 8-bit hardware, could only display a single background layer. There are some common techniques developers used to present a sense of multiple background layers in spite of this limitation; we'll go through a few of them using an example from my new Gameboy Color game, Depths of Europa.