Overall the game is great and one can spend a lot of time trying to win.
Some aspects could be improved for instance the camera is slow to track the player this cause that one has to move very carfull and slow, some enemys are fast and as the camera is slow it is dificult to anticipate the enemys.
Level design, animatios and sounds are great, and i loved that is not the same level each time that you lose, maybe adding some health recovery tool would be a good detail to avoid losing to fast.
Having to select the upgrades every level its a great idea, but dificult to nkow what is the diference between the four posible options because they look the same.