The atmosphere of Tom's apartment is, at first, described like a normal, functional apartment. It isn't until Tom is out on the ledge that he begins to contrast the two settings (apartment and ledge). After he is overcome with fear of falling, he must battle his fears and effectively, mind over matter (logic over emotion), will his way back to his window by taking small steps. It is here that Tom is fully aware of the differences between these two worlds. The ledge is literally living on the edge, always in danger. The bulk of this story is comprised of the descriptions of Tom trying to manage his terror while out on the ledge. The apartment is now a sanctuary for him, a place of safety and security.
He had a sudden mental picture of his apartment on just the other side of this wall--warm, cheerful, incredibly spacious. And he saw himself striding through it lying down on the floor on his back, arms spread wide, reveling in its unbelievable security.