Medium. I pretty much walked through the 1st 6 missions without dying (probably took longer than average, but always finished strongly). Mission 7 is a different story though. I get the wind trap up, the refinery built, and then the enemy storms me. Even if I fight off the 1st wave, and manage to get a heavy factory (or light, or 2nd refinery, or whatever) up, the bad guys just keep coming. It's kinda sad to watch, really.