Fantastic! I can't really say much about this because I can't really find anything wrong with it! Good work. Love the audio, visuals, and I found the increasing difficulty very fun.
One thing I would highly recommend is more keybindings and customizable keybinds.
Additionally, when introducing a new slot there should be some more time leniency to learn how the slot works.
Perhaps a level selector to go back to levels?
Also a level editor would be extremely cool!
There is some potential for expanding the game, but this existing product is already well polished and very fun.