As time passed, standardisation set in. Creatures like mongrelmen, originally created to play specific roles in specific scenarios, were added to the generic D&D repertoire: no longer just one particular remnant population in one particular forbidden city, but a monster race who could potentially turn up anywhere. One-off oddities like the Froghemoth were rewritten as species. It became accepted that the swamps of all D&D worlds contained populations of bullywugs and lizard men, just as all D&D underdarks had populations of derro and duregar and drow. You were doing something noteworthy if you didn't include them.
What had been a tendency in the AD&D days became official policy with third edition. With everything suddenly available as a PC option, everything had to be everywhere - because otherwise, what would you do if somebody wanted to play one? Every single book came with a long list of new races, classes, and prestige classes, each of them trailing a sad little paragraph about 'Illumians in the world', or whatever, which supposedly told you how to integrate them into your campaign setting. Default D&D-land became a place where dozens or hundreds of intelligent species rubbed shoulders on the streets of every major city, practising dozens of different forms of magic (each with their own guilds and academies), and worshipping hundreds of different gods (each with their own churches). It got crowded.
|Yes, Sigil was great. But surely not every D&D city should have to be like it!|
The trouble with this kind of 'top-down' approach, where every race and class and god and form of magic is assumed to be more-or-less universal, is that it gives each individual addition a very heavy 'footprint' on the setting. If the same gods are worshipped almost everywhere, then adding a god means adding a new temple and a new religious order to almost every city. If all your monsters are widespread species rather than one-off freaks of nature, then adding a monster means finding a place for it in your ecosystem - and, if it's intelligent, in your cities and cultures as well. But more isn't always more: and while adding a goblin ghetto to your human city could lead to some interesting world-building, if it's just one of twenty-seven non-human enclaves scattered around the city's outskirts then it shrinks from something important and noteworthy into just being part of a long list of token background elements.
Recently, I've been increasingly moving away from this sort of model, in favour of one in which most monster populations, divine cults, schools of magic, and so on are assumed to be intensely local. Maybe that shrine to the Queen of Storms up on the mountaintop is the only place in the world sacred to her, and the three old men who tend it are her entire priesthood, and no-one outside this valley has even heard of her. Maybe the lizard-man tribe who live in this swamp are the only lizard-men in the world, the result of magical meddling by some long-dead magician who once inhabited these lands. Yes, if your PCs kill them all, that means there are no more lizard-men - but so what? It's not like you're in any danger of running out of monsters...
There are a few things I like about this approach. It lets me use enormous numbers of different monsters, divinities, and so on within the same campaign setting, without making the world feel overcrowded: sure, there may be a hundred-odd intelligent races, but they live in a hundred different places, rather than all jostling together through the streets of every major settlement. It provides an easy way of differentiating areas: a wood full of trolls is going to be a very different sort of place to a wood full of hobgoblins, and can fulfil a very different role in the campaign world, rather than both of them just being 'generic monster-haunted fantasy woodlands'. It means that each thing is much more rooted in the campaign world: gnolls go from being 'one of a dozen annoying low-HD creatures who populate wandering monster tables' to 'those creatures which inhabit the badlands east of the City of Fallen Spires', with all the specific resonances and relevances that go with that. Perhaps above all, it preserves a sense of strangeness, of never knowing what's over the next hill or what might be living in the next valley. Top-down settings feel familiar and cosmopolitan: everywhere you go, you'll encounter the same creatures, the same religions, the same magical traditions. But more local settings can be much more mysterious, with the PCs genuinely not knowing whether, say, ettercap exist in this campaign world until they actually happen to encounter one.
|'Bugbears can only thrive under very specific environmental conditions...'|
This sort of intensely local setting design obviously fits in with more sword-and-sorcery style settings, where the world is full of isolated pockets of strangeness, rather than the more 'joined-up' worlds typical of high fantasy settings; but I think that it can also be used to lend settings a more down-to-earth, folkloric, quasi-historical sensibility. In most folkloric traditions, every moor or forest is associated with its own specific supernatural denizens: Black Annis lived in the Dane Hills, the Yeth Hounds lived in Wistman's Wood, and so on. Entire pagan mythologies, with their own distinctive pantheons, existed in regions which in many campaign settings would only be a dozen or so hexes across. (How much space would Wales or Lithuania take up on your campaign map?) The traditional, 'naturalistic' D&D approach acts to flatten out those regional variations, collapsing all these distinctive figures into one race of trolls, one race of hags, one pantheon of gods, and so on. But if you want a world which is at once more varied and more grounded, I think there might be something to be said for thinking of adventure locations as unique geographical and cultural microclimates, featuring creatures and gods and forms of magic entirely unknown elsewhere. Don't feel you have to pick just one of D&D's many, many takes on, say, fish-men: take them all and put them in different bodies of water. Go small-scale. Go local. Zoom in for a change!