On the run down/terrorist side of the map go to the underground closest to the red room. Aim slightly diagonal (towards the corner of the low wall) on top of the underground exit. Sprintjump to it and pound away at the 'A' botton when near the wall. You'll pull yourself up. Jump onto the corner of the building next to that wall, and walk up the broken wall to your right. Jump into the next building on the next floor. There will be 2 broken out windows with sandbags in them. Sprintjump out the one on the right to the building across the street, and try to land just right of the crumbling part of the floor. Walk to the wall on your left and climb it. You can go into another building from here. Go to the broken wall on your left. Climb up this and go on top of the wall until you're near 2 trees. Sprintjump into the building across the small courtyard. No one will look in here, so it's a great ambush point, but it's inescapeable, and suicide's the only way out.
P.S. All the text in red is what I've seen other players do during online matches, the rest was through trial and error
P.S.S. You can stop in any of these buildings, they all offer exelent fireing points on many different areas