Good game and cool idea starting at the end. I found the first boss difficult with too much HP, but it's probably my fault because I suck when the controls require 2 hands at the keyboard.
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They are good, but in my opinion you should try to be coherent with the different assets, some of them have different pixel sizes, for example when comparing the boss ship against the player, and the 'x' circles that the boss shooted were smooth, not pixel art, conflicting with the other art. Except for this, nice sprites.