if they know where you are, they can shoot through the ground and maybe kill you.
in certain areas you can throw grenades through the ground, too. you can shoot them through the ground without them knowing where you are. then, you can see other players walking above you. then, turn left until you're facing your back to the brown building. you must move very slowly in the wall walk towards the stairs while still in the wall. keep doing this and you will eventually end up inside the white wall. when you get towards one end, turn around and walk against the wall towards the other end while going forward. once you get towards the stairs that lead a little upwards to the brown building, start running against the white wall. First go towards the brown building from the front instead of the sides and back.