In the back of the stage in the destroyed building there are 4 little sections that you can go into. from all 4 sides you can do this but it is better to do the ones toward the boxes and trees so you dont make yourself more visible to other players. there are windows in these little rooms, jump toward them with some momentum and at the right time press crouch (B) to get inside the window. when you are in you will see a little gray ledge right below the window you have to SLOWLY drop your self on it by walking out the window. once you are on you can walk around the whole building but you have to be very careful or else youll just fall off. its a good place to snipe since not that many people know about it. Good Luck