The campaign has three difficulties, and you must complete the entire campaign to move on to the next difficulty. It does not have a certain number of levels, but rather 24 threat levels. When you select a threat level, a map is allegedly randomly generated with the selected threat level. Having said that, I have played the same threat level more than once, and found myself on identical maps, so perhaps I’m missing something. Each threat level has different enemy levels. The first level has enemies with level 0-4, the second has levels 2-6, and so on. They are named in the same manner as older games: rather than going 1, 2, 3, 4, 5, they follow the pattern 1-1, 1-2, 1-3, 1-4, 2-1. As the first number in the level changes, the level becomes significantly more difficult, and the level range increases by four rather than two. You must complete a level before moving on to the next. I’m not sure about this next bit, but it seems to me that you have to complete the level on single player before you can move on. This really serves to discourage co-operative gameplay. An annoying remnant from the co-op gameplay that remains in the single player is the inability to pause. I can understand this when there are other people playing with you, but not when you’re playing alone.

preview here