I've always lived inside the walls.
I asked the big people, including my mommy and daddy, but they always told me they would let me know when I was old enough. I listen, though. I often hear weird sounds and bangs. When people go missing, it is often because of "them."
On a hot day in my tenth summer, my daddy told me it was time. We walked towards the wall and they dropped a ladder. We climbed up.
The weird sounds were really loud, now. I looked over the wall to the large world outside. What I noticed was the gray skin, the gnashing teeth, and the wounds in their skin.
It was then that I realizd that the "them" were once us and they were hungry.
(This story is credited to a person called Richard.)