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brohocho,you can replay levels to earn upgrades.Just click menu and click the level you want to repeat.I'm now on level 35.Five to go:)Not a bad game.
A tip to get to the second third level,be accurate and don't use mines or airstrikes.They will stockpile and when you have a lot,blast the hell out of everything:)
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Hmmm, not sure about this game. It would probably be more enjoyable if when a level is failed rather than "game over" you could simply try again - especially for levels you are repeating to try and earn the upgrades. Without this there doesn't seem much chance of me ever seeing many of the 40 levels and 27 ranks.