These are the bedrock of how I make this fantasy setting work; everything else follows as realistically (verisimilitudinously?) as can be dreamt up. (While always maintaining a bias toward entertainment.)
Pillar 0: Gods are real. Not all of the gods that people proclaim as real are real, but there is a pantheon and they do interact with peoples’ lives on a daily basis (even if indirectly).
Pillar 1: Magic is real. It can be big, flashy, and everywhere; it can be hated, subversive, and feared; or it can be storied and rare. But it is never perfectly reliable and predictable. If magic produced the same result for any person who performed the identical actions, it would no longer be magic, but science. And that science would quickly become technology. Maybe there’s no predicting who can do magic or why; maybe it’s fundamentally unreliable and can’t be consistently replicated; maybe anyone can do it but the effects produced vary. But there’s something fundamentally unscientific about magic.
Pillar 1a: Magic does not change the human heart. Human impulses remain the basis of the dramatic, and the dramatic is what we’re here for. We’re all still the same people, telling endless iterations of the same few stories, just through different instrumentalities.
Pillar 2: Life is hard. Crops fail, and then the lord comes and levies the rest. The pox smolders through the villages, then returns the next year. Rains flood out cartroads and most labor from sunup to sundown to keep their families going. And that’s all with brigands and chevauchees and whatever eldritch creatures roam the badlands. Sometimes the dead crawl out of the ground.
Pillar 3: Violence is endemic and terrifying. Sure, ancient enmities between dwarves and goblins, orcish hordes, all that. Political and personal violence is sadly commonplace. But what’s most vicious is that it’s really about ecological pressures. With a variety of sentient humanoids all sharing overlapping, if not identical, ecological niches, competition for resources is constant and ugly. Eventually one race or another will likely win out, but that hasn’t happened yet, and it will be a red day when it does.
Pillar 4: Everything has an ecology. Most everything eats. Everyone lives somewhere. Some grow in the day and some hunt at night. Apex predators—of which there are many—tend to be individuals, but everything else forms families, packs, or colonies. Everything has a place. This is pretty easy for all the things we’re used to, but creates interesting implications for those we’re not. Orcs form warbands, but also have oatfields and settlements and mills and harvest festivals. Dragons have a large-enough ecological footprint as to warp the landscape around them. And that catoblepas has to be doing something in the 99.99% of the time that a paladin isn’t trying to slay it.
Pillar 4a: There’s a reason for that dungeon down there, and that reason isn’t “a wizard did it.” Excavating is hard work, and the use must justify the cost. It might be some beast’s burrow, or a tin mine, or the tomb of the magnificent and mighty, or an irrigation system. But there’s some damn reason.
Pillar 5: All evil is specific. Nobody wakes up in the morning and thinks, “I’m going to go be wicked today.” Evil can be clever, or delusional, or callous, or lazy, or expedient, or even simply desperate. But there’s always a reason. Creatures always do evil for a goal. To gain something, or avoid losing something, something real or imagined. Evil can use good means to bad ends, or bad means to good ends, or bad means to bad ends. There’s always a reason, even if you can’t understand it from a distance.
Pillar 6: Almost everything is mysterious. There’s some impossibility, but it’s mostly ignorance and lack of communication. Sure, it would be crazy to believe that outlander’s talk of giraffes with ten-foot necks topped with a man’s head, but did you hear of the man in the cave with a bull’s head? People need to feel like they know the reason why things happen, and readily fill gaps in their knowledge with supposition and imagination. What are those fires that burn in the sky, and why does the world freeze every year? That toddler fell down the village well, but what child would just do that? Surely it was the wickedness of Goodie Kerrand, who lives by herself and quarreled with the mother that very day. When you’re like as not to never venture more than three leagues from where you were born, it’s easy to dream up what might exist out there in the beyond. The average person’s horizons hew very close indeed.
Pillar 6a: Perfect information kills plots. And Identify can’t identify everything about a hexed item, even if the rules say it can. How could it? How could an in-game spell tell an in-game character what the “+2” on a Kris +2 even means?
Pillar 7: PCs are exceptional. As in, exceptions to the normal expectations. And we’re not talking about “roll 5d6, drop lowest two.” We’re talking about people who buck the world and do what the vast majority cannot or will not. We call them “adventurers,” but no one else does. Highwaymen, blackguards, outlaws. Tomb raiders, freebooters, marauders. Looters, rievers, opportunists. Men and women like this are antithetical to order, to civilization. And that makes the common person—and those who rule over them—uneasy. In a dangerous world where stability is paramount, adventurers are a destabilizing force. Occasionally useful, maybe, but no one ever smiles when they come to town. And that’s just the business of carrying swords and killing things for private gain—leaving aside calling thunderbolts from the heavens or shooting inerrant arrows from the fingertips.