This is how to do it the one-player way. Play through the first section of the game in which you must "hold the fort" and defend against the covenant attacking you. After that is done, you will walk through a broken down area on your way to the next part of the town. At this point, you should see a wall blown into two halves. From where you came from looking into the next area, jump onto the left wall at the bottom. Now, edge over to the right as far as possible. Run and jump over to the right wall level 2. Now, jump to left wall level 2. Walk up the plank, and jump onto the rafter. From here, jump into the rubble, and cross over to the right side of the rubble. Now, jump onto the rafter near you, and jump onto the right rubble wall. From here, jump ontop of the second story of the rubble. Now, you can jump on top of some of the small buildings.
From here, go as far as you can to the left without falling off. Jump over onto the building in the area you were going to in the middle of the town (about). From here, jump to the right, and go until you see a bunch of vents. Jump onto vent #2, and then jump onto the nearest balcony. Jump to the next balcony, then make a sharp right turn and crouch. If you are lucky, you will either land on the ruins or the balcony. If you are on the balcony, jump to the rubble. Now, climb up the part sticking up on the ruins, and jump onto the top of the building. You are now outside of the level!
Submitted by: Thor