There were no walls around black towers. But if you entered one without the proper eyeballs, you would be hugged in a darkness so profound it froze your limbs. In that darkness, they said, your body vanished. You couldn't see a fingertip held an inch from your eye, not to mention lights that might be burning inside the tower you entered. You were a ghost floating through the blackest nebula in space.

That way, thieves, vandals and panhandlers had to stay outside. They had trouble buying the proper eyeballs.