Beyond the Wall

Two hundred years ago, there was a war. This war ended all the states and cities in the world, at least that is what they tell us. They tell us that we are the only ones left, so we should be thankful for our city. However, there is this wall on the outskirts of this place. If we are the only humanity left, then why is there a wall? If we are the only ones left there would not be a wall.