hmm
i could not finish this game, it gets way too boring.
my suggestion is that maybe you could take out a stage or two, i think 3 stages would be better per level. maybe you should consider staying at 5 levels per area too rather than increasing them as the game progresses, that way the game moves at a faster pace and you don't lose any quality in your game.
just a suggestion.
the bosses were fun though, good job on them.