But what if your wall around the island extends upwards, to the starry sky? What if it stretches miles high, a massive needle jutting out from the dark blue sea? What if all other humans on earth are zombified, and the three inside the tower are all that survive?
What if they learn to build terraces on the wall and farms high in the air? What if their children and their children's children extend the tower even further? What if, hundreds of years from now, entire microcountries have sprung up inside The Tower?
The highest levels have been abandoned for hundreds of years, and jutting terraces make nearly any upward view a blank scene of rock. The inhabitants, having long since lost knowledge of the outside world, believe that The Tower is all that exists. They press forward, higher and higher, striving against each other to reach the sky.
Some day, they will make it. They will look up, and they will see the same stars that you and I, their distant relatives from ancient times, look upon today. They will see the dark, inky blackness of night, twinkling with innumerable points of crystalline light. They will see. They will wonder about the past. They will look down over the top of the wall, and see the massive stone needle that they have spent their entire lives inside of, stretching down into a belt of blue haze, faintly luminous in the dampened light of the stars.
They will heft their climbing gear, swing over the edge, and descend. And humanity's life outside will begin anew. It will be beautiful.