The sword & planet genre is relatively new for me. I have not read the Barsoom novels. I did not know about Expedition to the Barrier Peaks before I discovered the OSR a few months ago. My fantasy has long been spiced with steampunk elements, reminiscent of the style of Final Fantasy IV & Final Fantasy VI, but I have usually pictured those as products of the current society (like magitech in Final Fantasy VI) rather than remnants of ancient advanced civilizations. I watched and enjoyed the original Stargate when it came out, but I don’t think it had much influence on my gaming. Despite that lack of experience, the science-fantasy elements of Dwimmermount’s Red Elves of Areon, modules like The Tower of the Stargazer, and the setting of Carcosa, have made me think again about introducing ancient laser rifles and robots to my fantasy games.
How would that interact with the rest of the setting though? I like my fantasy setting to feel almost radically unknown (though not necessarily surreal or gratuitously strange, though there is a place for that). Someone living in ancient Babylon, for example, would really have no idea how the universe worked, in terms of metaphysics, cosmology, and possibilities. Does the ocean flow over the edge of the world? Maybe! Using tropes from the sword & planet genre seems to automatically import all kinds of assumptions, such as, off the top of my head: spherical planets, gravity, and outer space being a void. I see this as potentially working at cross-purposes with the sense of the fantastic, even if players are not put off by the genre-bending. One way to handle this would be to just not worry at all about consistency at the beginning, and come up with interesting explanations after the fact, in the same way that one might handle a seemingly contradictory or confusing result when using a set of random tables. There is something pleasingly classical about this approach. Somehow later rationalization does not seem satisfactory in this case though.
I have been mulling over a possible solution to this dilemma: the nature of reality varies based on verticality. As characters venture deeper underground, the world becomes less mundane, and as they ascend into the heavens it becomes more systematic. In its deepest reaches, the underworld bleeds into the afterlife, the shared reality of dreams, and other weirder places. Even geometry and gravity may cease to behave as expected.
This doesn’t need to be overly schematized or communicated to players, it just serves as a set of conceptual guidelines for the referee. And it reflects many interesting dichotomies, in game tradition, mythology, and philosophy.
In the earliest versions of D&D, dungeon level was explicitly associated with threat level. It got more dangerous as you went down. This just made sense based on the needs of the game, but has since become enshrined in just about everything that has been influenced by D&D. In Greek mythology, Olympus is up, Hades is down. In Aristotelian cosmology, heavenly bodies are made of perfect substances, whereas all matter on the earth is corruptible, and this was carried over to the medieval worldview (through Ptolemy) and the thoroughly Christianized. There are many inspirational pages up on the web about medieval cosmology.