OK, you're in an open snowy area, with a building in the middle. Go to the other side - left around the building. When you go down the ramp, there are marines there waiting for you. Pick them up and carry through. Soon you'll hit an open area with turrets. Get to the door on the right. They'll stay there and won't come in.

Work your way around to the left and take the elevator up. There's another circle room. Run right, and you're up on a bridge. Hit the sleeping guys with the butt of your gun to take them all out without anyone knowing you're there. Then cross on the left so you don't get blasted by enemies on the other bridge. There's an invisible guy at the end to kill.

Another circle room, go to the right. You hit a corridor. Yup, another circle room, head right. Then another bridge. Another circle room, head right. Another corridor. Another circle room, head right. Another corridor, and a final circle room. And this level is done. Master Halo Walkthrough