I recently finished the original 5-book Well World series by Jack Chalker.
The Well World is essentially a global zoo created by an ancient race’s superscience. The world’s surface is evenly compartmentalized by hexes that maintain specific atmospheric compositions and technological limitations, and populated by a unique sentient race. The southern hemisphere’s hexes have a bewildering variety of alien life, but all are carbon-based, living in mostly familiar atmospheric conditions. The northern hemisphere hosts the more bizarre aliens and exotic “worlds”.
The most fascinating part of the series is, of course, the Well World. The hexes, while perfectly maintaining their unique environment, are permeable. There is some cross-border travel and trade, but mostly the races keep to themselves.
Each hex also has a technological development ”cap” which frustrates cross-hex conquest: A highly-technological race invading a non-tech hex will find their tools to be mostly inert, for example. But there are clever workarounds which make for fun reading.
The ancient race, the Markovians, not only created the Well World, but also ruins scattered around other systems. Many had portals that would draw in unsuspecting individuals, transporting them to the Well World, and then sending them to a hex, transformed into one of the local inhabitants while still possessing memories of their previous life.
I felt this was the most compelling part of the tale: The Well World, its unique biomes, and the interaction of its races. Unfortunately, after the first two books, Chalker spends too much time away from the Well World, instead detailing the Com – the local human star civilization – which is less interesting. There are some bright spots here, such as a super sentient computer, and a frightening alien race composed of microscopic life. But the series would find its stride each time it returned to the Well World.
There is an obscure roleplaying game based on this setting, “Midnight at the Well of Souls” (http://www.waynesbooks.com/ringworld.html#mws), authorized by Jack Chalker. Its strengths and weaknesses echo the fiction series in my opinion, best when it focuses on the Well World, but undercut by wasting ink on starship blueprints and yet another space combat system.