There are 3 Dimensional and 2D dimensional realities. In most campaigns, systems are contained in Crystal Spheres and Wild Space. Some are more or less real Universe types. But in a 2D Reality, all matter and substance are on one flat Prime Material Plane. There are numerous areas isolated by distance or elements. Oceans mostly, but also cliffs and peaks of harsh terrain, the Underdark, lava seas, permanent powerful storms, large regions of desert or ice, and such. The most common area is a group of continents alone in an Endless Sea.

This Prime Material Plane has collided or attached Itself to a 3D Prime Material Plane. At the confluence, there is a pocket mini-reality where all the mass of the Prime Material Plane exists on the surface or interior Ten Thousand Miles Squared Cube. Each Outer Square contains a differently themed Campaign Setting. Travel between theses areas is rare and difficult. The area of gravity fluctuation makes flight impossible. Subterranean routes are more common.

While the Alternate Reality Prime Material Plane's Outer Squares are relatively stable, deep underground it fluctuates. Sometimes, random groups of creatures or terrain arrive from the adjoining Prime Material Planes. There does seem to be a capricious intelligence guiding this. Setting up conflicts and challenges, entire vast entire Underdark caverns with differing ecosystems and challenges. Connections between this Deepdark at the core and the Underdark caverns near the surface are rare and perilous. Over the centuries, a group of survivors of these abductions and the Deepdark Itself, have formed a loose association and economy. Trading points were established to trade items and wealth for much needed supplies. The largest of these meeting points, named Rendezvous, has grown into permanent and sizable town. The population is almost wholly made up of kidnapped adventures and their descendants. A rough 'Frontier Justice' exists there. Rendezvous is neutral ground, any attack on a resident or visitor is punished by immediate death or exile from town. There are 12 'Pillars' of Rendezvous. These are supposed to be the most powerful residents of the town, but some are opposed to the system or don't want the hassle of an appointment. Regardless, the 12 Pillars are skilled, high level adventurers who are on call at all times and expected to defend the residents, act as sheriff or general, and use personal authority and persuasion to resolve conflicts peacefully among the population.

The town of Rendezvous acts as a base for parties looking for a way back to their home reality. Usually for connections to the difficult to reach surfaces. Though many have left on these quests, no word of success has ever reached Rendezvous.