This one from Saberweaver:

To get on top of Ivory Tower...go to the Overshield spot with at least three people in a custom game. There is a little square to the side of the Overshield...have one guy stand in the left back corner (if he is facing the Overshield). Have your Primary weapon as Rockets, and Secondary as Sword. Back up about a few feet away from the person in the corner, hit the melee button using the Rocket, and it should lunge you forward (but not hitting the guy in the corner). After you hit the button, immediately hold right and you will fall through the map (this takes some practice). Right after you start to fall through, hold back like crazy and you will land on this long grey, thin ledge below the map (also, takes some practice). Have another guy do the same thing. Go all the way to the left, next to the column that's connected against the wall, and Sword cancel all the way to the top of the map! Now you can walk and basically see everything that's going on below you!