I found this to be a maximum difficulty game. Even though I was able to complete it, it was more of hair pulling experience, than something I would describe as "fun". I would pay for a Part 2, if the game offered variable difficulty settings, and/or the ability to build up my characters through some other means such as side quests or a grinding mechanism. I only earned enough money to buy a bare fraction of the upgrades. Likewise the skill purchases lagged behind and left me utterly frustrated. And yes I completed 95% of the kills, and about 80% levels completed without death, and gamed the save system, and utilized the environment. I believe this game would gain a wider audience if it had a middle ground between the casual player and the hardcore player. Dear Developers, it you look at the comments, you will see the vast majority saying the same thing. Go ahead and take my money next time (maybe).
Like others have said, the game fails to award any stars for the completing the last level, Dark Tower. This makes it impossible to get last the badge.
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Vulnerability means you take more damage. It's a tradeoff for "Fool's" items.