Reading below, I'm seeing that vertical movement was cut for performance issues. I am seeing after the first boss, there seems to be a bit of stage design that really expects that vertical movement, notably narrow pathways with turrets a little past it.
The early levels were very empty, I'm unsure if that was to give the player time to figure out the controls or if GBStudio didn't quite have the efficiency to do more.
The soundtrack is one of the best I've encountered this jam.
A shmup is a very GameBoy thing to make, with R-Type coming to mind as a rather prominent one that was ported to it when the console was contemporary.
A very good entry into this jam.