The Great Road
The Great Road ain’t really a road and it ain’t all that great, but that’s what folks call it: the Great Road, the Ring. Fact is, it’s an idea more than a thing. There’s actually a road on some planes, but mostly it’s the thought of the Ring, the grand union of all the Outer Planes, that poets sing about. What it really is is a string of portals, permanent and unchanging, that link each Outer Plane to its adjacent fellows. Now, if a berk had the years and the compliance of the fiends in his path, he just might be able to walk the whole thing in sixteen lifetimes, but, then, who’d want to?
Each portal on the Great Road exists as an arch, like the portals of Sigil. However, looking through one of the arches of the Ring, a being can see his destination on the other side. Passing through doesn’t require a key, either. All a being’s got to do is step through the arch.
Towns, forts, encampments, even barricades spring up around the Great Road’s portals. On the Upper Planes, they’re usually used for trade and commerce, but on the Lower Planes these towns are staging points for the endless Blood War. These are dangerous places to pass through - a stranger here could get himself gutted as a spy or dragooned into the fiends' ranks. Most folks don’t go there without a powerful reason.
Each Outer Plane has a permanent portal that leads to the Outlands, too. Towns sprout up around the Outlander end, like Plague-Mort, Ribcage, Glorium, and Automata. But even though these burgs sit on the Outlands, their character is very much that of the plane they watch. Ribcage, which lies at the portal to Baator, is thick with pestilence and horror, and Glorium, near Ysgard, is filled with noble virtue, and so on. These are places where low-level adventurers can get a taste of the planes with a little less risk involved.
Note: The Great Road originally appeared in the Planescape AD&D campaign setting from TSR. Its use here is for the purposes of providing context for the campaign only.