All in all an enjoyable game, with tight enough controls and a good visual style.
My main complaint is one I have with several games that have upgrade systems: Putting level limits on the early extra challenges just makes me feel cheated of my grinding time, and makes me not play them, rather than adding difficulty. Difficulty is not by definition bad, but the whole point of upgrade systems is that you feel more and more powerful as you progress, and having that power artificially taken away is off-putting.
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