The story is set in a walled-off town. The towering structures that seal the people in are so tall that no one can see over and the billowing smoke from the thousands of chimneys forever obscure the sky.
Chimney Town is surrounded by a wall that's 4,000 meters tall (that's about 2.5 miles high). As the name suggests, the walled-off town is choked with chimneys and smoke is ever present. In fact, people have forgotten what the sky looks like.