I love the twist in the tutorial !
Pretty fun game, with a lot of different enemies, and a lot of feedback on everything you do, well done ! Especially for the camera that seems to zoom in and out depending on the distance of the enemies, which is really well made.
The upgrade system has flaws, as it was pointed by others, but it is well made enough to give variety and make the player play again to see everything.
To sum up, really cool game !