Only black mark is the fact that you can't move left or right whilst holding up to climb ladders. Seriously cripples my mobility, and not in a "part of the challenge" kind of way.
I bet to beat extreme mode you have to build up a defense, then sell everything and buy the higher-powered weapons you get later in the game. I'm going to try to get a ton of laser rockets going, then sell (S key) and buy sniper rifles.
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