MECHANUS -- LAWFUL NEUTRAL
Gate-town: Automata. It's the most ordered and perfect burg of the Outlands. It seems a machine more than a living place and it's full of lawful, ordered, overly organized bashers. Inside its rectangular city walls the town is ordered as a cartographer's study. The gate: it's located in the center of the town and it's a great washer.
This is the home of law, order and clockwork precision. It is also the home of the modrons, native beings that maintain and guard the plane and its workings. In fact, the plane consists of a series of endless gears, forever turning and driving something.
The inhabitants of Mechanus make their homes on the very cogs that make up the plane. The gears don't have vegetation or native wildlife, but plenty of life thrives amid the machinery. Regulus is home to the supreme modron, Primus. It may be that Regulus is a layer itself, or just a realm, but it's impossible to tell. Mechanus is one constantly turning clockwork and the boundaries between layers and realms aren't always defined.
The modrons, naturally, maintain a strict hierarchy. They are divided into well organized ranks; more intelligent and powerful are the modrons, higher is their rank and fewer they are in number. A planewalker needs to know one more thing about the modrons and that is the Great March. Approximately every 289 years of Sigil (17 cycles on Mechanus), the modrons marshal a huge horde and leave their plane to march around the Outer Planes. No one knows why they march, especially since so few of them live to see the end of the march.
Within this realm of perfect law, exist few places for less orderly types. One of them is Haven; the city appears to be a huge cube, walled on all six sides. To most folks' eyes it's not a wild chaotic town, but on Mechanus it's an intolerable cacophony of disruption and lawlessness. It's a good place for a planewalker to take refuge, if she can convince a sponsor (from among the lawful permanent inhabitants of the city) to let her into the city.