The walls are not of stone or steel or wood, but of mind. Buildings cease to be built beyond the boundary. The unruly orchestra of the human song would fade if one were to advance past it. One dares not, except in gas-eating wheeled city-extensions.
Above the boundary, thousands of light-years distant, the stars shout down to us that they exist, and we marvel at their beauty while insulating ourselves from their presence. The gentle woodwinds and percussion of nature comfort the lonely stars, for we sing to them no longer.
Past that ebon veil, pageants of myth are acted out by the gods according to scripts penned by the depths of our minds. Were we to extinguish the city lights and hush the city noises, we might get a glimpse of these performances.
What a terrifying prospect.