Well I've played a few levels and it looks to be a funny game. Until now I only think that the coins earned should be earned only once per level/difficulty, as it is very easy to upgrade if we repeat the first level. What I mean is, for ex: 3 coins on normal, 5 on hard and the max you could earn from that level would be the 5 coins (one time). Just an opinion of course.
Good game!
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 "Champions of chaos"){kind=icon}