The Strange Worlds tunnels may or may not have an end, but they certainly have a beginning. Each tunnel begins with a Gate and behind the Gate is Nexus City.
Nobody knows how old Nexus City is. A hundred and seventy-five years ago, when the tunnels were rediscovered, Nexus City had been in place, the buildings empty, silent, and dusty. The general consensus among forensic archeologists is that the dust is approximately two thousand years old. Some theorized that it is no coincidence that this period more or less coincides with the rise of Emperor Heinlein.
There is also evidence that small groups had access to Door tools and over the centuries had periodically explored the tunnels and Nexus City. Be that as it may, the city's revival corresponded to the general rediscovery of the Strange Worlds' tunnels by citizens from Blue. As more and more doors were opened, and as more and more visitors moved from door to door and tunnel to tunnel, Nexus City's importance as a commercial, cultural, academic, and athletic center grew.
Nexus City exists under a triangular dome. The architecture is as varied as the inhabitants. A skyscraper rises next to a rough hewn castle, both facing a park full of fountains and trees. A factory can be connected directly to an immense tree house complex and what could only be a train station. There aren't very many streets -- the buildings are just plopped down at random.
Air, water, heat, light, and electrical power come from mysterious sources. Climate is maintained at Earth sea level, 72 degrees Fahrenheit, and 40% relative humidity. With a few exceptions, the buildings are scaled to human size. Whoever built Nexus City (and the tunnels?) was probably had humans in mind.
Nexus City is a triangle about 10,000 yards (about 2 miles) to the side. (1 mile = 5200 feet.)
There are several hundreds of building in Nexus City buildings and they range from single room bunkers to multi-block shopping malls and high-rise living accommodations. Here's a list of some of the locations mentioned in Amanda Harp's Day Logs.
Miskatonic University Alumni Association
Notsie Zombies Headquarters
Quantum Man Memorial
Red Embassy (Heinlein's Imperial Consulate)
Zone Pioneer Headquarters
Of course the largest building is the Strange Worlds Arena (A 2,000 yard triangle that takes up most of the center of Nexus City).