@Gahan The region boss modifier adds flat stats to the enemy. In the early levels they'll easily one hit you but as you progress they fall more in line with the average enemy.
Since the enemies level with you it feels like you get weaker as you level and the premium content really hurts when we expect to play for free. That said, the game is still well worthy of a 5/5.
Woo! Got brilliant defense! The trick is to be careful on the last level, cut wood sparingly, max out your mana regen/max mana and build a bunch of mana regen crypts then try to hit as many thieves as possible with your meteor. Remember to use SHIFT when casting!
The only downgrade ive seen in this game compared to any of the other games is elements are no longer related to special abilities. Poison no longer causes a dot and such.
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You can complete the whole game and get all of the achievements without the paid content. It just gives you more options.