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I actually love the "drum concept." That's what makes the game so special. Without it, Bongo Boom Battlegrounds would be no different than some other flash games here on NG.
I'd change it slightly, though. At some point my fully trained and upgraded basic troops were as good (if not better) as the tier II/III advanced troops. You could make the game a little more difficult and the advanced troops a bit stronger or let us upgrade them at least once.
More things could be added, but that would make the game a bit too complex and probably hard to play. So, I'd rather see a sequel with minor changes, and maybe then the 3rd version with some complexity added.
Great game, by the way.