Metaphysical / Paranormal event
The Vault doors open.
On the 6th day of the 2nd Moon 20 B.F, the centerpiece in the main hall opened up, revealing a long flight of stairs leading upward. The residents, having organised a semblance of leadership amongst themselves, sent people to investigate what lay on the other side. The answer was shocking - towering walls surrounding an open-flat area whereby streets and key infrastructure were all built and in-place. The only thing missing were buildings. Soon, people started to venture out further, and set out to build up the city. It took them weeks and weeks of planning before a consensus was reached regarding the city zoning.