To finish the game with 3 star rating can be challenging in the later levels. I used all the points on all tower and ability upgrades, except arrow tower. The arrow tower has no real benefit compaired to mage tower. Then I used every remaining point in the hero unit. Use your hero as efficient as possible, and you might want to keep your last tower (knights in my case) on minions only, else they might hop on the giants and let 1-2 minions pass now and then, u need atleast 18 'castlepoints' to score 3 stars.
Really fun game for as long as it lasts with nice graphics and atmosphere 5/5. Its not really hard but some levels might require trial and error (1-20). Getting the hard badge aint really hard either, just a little bit of patience on some levels. I finished the normal 2 packs with total of 139 stars. and just as EPR89 mentioned, try rebound shots as much as you can since that can easily give you the 4th bonus star on some levels.
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