This was originally a little thing we did in school, with only the very basics of what it is now. I tried to give it a sort of old-arcade-game feeling to it, which I think it has.
There are 8 levels and 4 bosses in total. If you don't understand how to play just do the tutorial.
You shoot a powerful shockwave with the space button, but only once per level.
You switch you normal bullets to rockets with the down button, and then switch back with the up button.