The whole last 30-60 minutes of Portal 2 was one of the best experiences of my gaming life. The only thing that I found odd was that when you look at the moon to shoot the portal, it appears that Wheatley's chamber is at the top of the facility (as in you're already at ground level). The problem comes 5 minutes later when GLaDOS starts taking you up to the surface, which at least consists of 15-25 stories more stories until you reach ground level. The only solution that I can think of is that Wheatley pulled his chamber up to ground level and then GLaDOS pulled it back down.