This guide will show you how to get over the walls of Arkham City and explore all the surrounding environment.
In order to make this trick work, you need to stand on a ledge or a billboard, then face toward the corner and start spamming the button to throw freeze grenades (RT on Xbox 360, L2 on PlayStation 3 and "5" on PC).
Batman will slowly gain altitude, the smog on Arkham City will disappear and, when high enough, you can safely "escape" from the walls of Arkham and explore Gotham City.
All the buildings, streets and cars you'll see are in low textures since they are purely decorative. You can't use the Grapnel Boost, so Glide and Dive Bomb to not lose too much altitude.
You can also explore Arkham Island.
Load the game again to get back in Arkham City or die by falling into the underside of game's world.
Thanks to MadMachinima for the guide. Check out his channel at http://www.youtube.com/MadMachinima
Game available on: Sony PlayStation 3, Microsoft Xbox 360 & PC Video recorded on: PC
For more info and videos, visit: ● http://www.youtube.com/BatmanArkhamVideos ● http://www.batman-arkham.com ● http://www.games-series.com