It's the shape of the board which generally determines whether a level is a pain in the arse or not - long thin sections of board being particularly troublesome. Some of the frustration is absorbed by the aforementioned special tiles, which build up various bonuses - items which destroy a number of tiles for you, extra time or resources, that sort of thing.
All this makes it a more varied game than Zoo Keeper. The execution is sloppier though: some of the tiles look annoyingly similar, and the accuracy of the touchscreen isn't what it should be.
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