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Few tips:
1.If you upgrade your mages to the max, they get extra damage for each additional mage tower you have. Even though after the 8-th tower you don't get any extra, if you build a lot of them, their damage significantly improves.
2.The archers have a passive skill which improves the attack range of all nearby towers. This combined with the artillery tower which makes splash damage to all enemies around is deadly.
3.The necromancer mage tower is good to built early in the game, so that it will accumulate more and more skeletons to block the way during the later waves.
Under rating threshold (hideshow)
Great follow to the first game. The load time though is terrible. I wonder if it is intentionally this way to encourage you to buy the mobile version.