It's a space elevator/strategic weapons system/factory/Internet backbone server/magical tower of the gods. It's a lot of things, and it's so big that not only do people live on it, but there are forests and farms growing on it. There's a staicase that can take you from the top to the bottom, but that means climbing for literally miles up into the sky. And you know, getting past the locked doors that almost nobody has clearance to open anymore.