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If you're a fan of the castle defense genre, but you like your games to be a little on the strange side, then Grove Keeper HD is probably for you. It's an inventive addition to the genre that's great to look at, and though it isn't perfect, it's probably worth your time.
The game has its own little world and mythology, involving a mystic force, a light and dark side of said force and the grove keepers that control it. There's an evil keeper threatening the balance of everything, and you play as the only other keeper who can stop him (basically). The story is very quirky, and the dialogue seems like it was written in a pre-teen AIM conversation. It was pretty annoying at first, but it grew on me, as it matches the tone of the rest of the game.
In the game, you move from location to location and have to defend makeshift forts from waves of enemies. As the keeper, you have control over nature and use it defensively while random archer dwarves kill enemies. You can block enemy projectiles with the stone magic, entangle them in plants that stop them, or blow them away with tornados. It's important to keep your archers alive, as they are your only means of killing enemies for nearly the entire game.
Eventually you gain a magic bar that's filled as you use powers, and you can use that magic bar in conjunction with any of your defensive attacks to devastate the enemy forces. There's an entire chapter dedicated to these powers, and it was easily the most fun part of the story. There is also a survival mode in which you play the story levels, but with infinite waves. This adds some extra challenge and variety. With lots of game center achievements as well as leader boards, there is always something to strive for.
Overall, I'd have to recommend this game based on the great art style and strangeness. It doesn't take itself too seriously, and that's refreshing. Of course, there is a cheaper, non iPad version, though I'd recommend the HD.