You start at point A. You try to get to point B. That's all there really is to it.
With some levels, you can go slightly off track, going around two different sides of the same building, but in the end, you always end up in the same place.
So yeah, it was quite linear. But, then again, so was Half Life. And Portal. And Prince of Persia. And Mass Effect 2 (At least the combat parts were). You see where I'm going with this...