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Size of the world: Well, undefined. I would say, 1000 miles from Wisloth to Breakwater.
World Style: Think Conan's Hyboria + Zothique + Dying Earth by Jack Vance + Newhon by Fritz Leiber + some Cthulhu mythos thrown in. Countless civilizations left their mark on the world, but most of it now lies in ruins. There is some post-apocalyptic flavor in this otherwise fantasy world, as some items and artifacts of ancient advanced technology can still be found here and there.
Races: No Tolkien, no gnomes!! It's more like humans (of course), plus degenerate humans (half-orcs), tieflings (touched by supernatural evil), and dwarves. Dwarves are the descendants of a slave race that was bred by serpentmen to toil in the mines in a distant antiquity. As such dwarves get bonus to hit against serpentmen and snake-creatures, not against goblins.
Classes: There is somewhere around here, a document to convert/use AD&D1e rules in a dark-age, dark sword& sorcery setting. I would take inspiration from it. World is grim, sorcery is corrupting, etc.
Religions: No pantheons as is usual with D&D settings. No Pelor, no Odin, no what not. Instead there is many cults. Think dark cults of obscure mad Cthulhu deities. See also a thread in The Workshop forum that discussed that, and gave examples of dark cults led by lamias, yuan-tis, whatever. In any case, such cults are almost never widespread, but occasional and independent of each others. Then, there is two organized religions:
1) The Issamnites, a Scarlet Brotherhood-inspired cult of fighting-disciples who will eradicate mutants, blasphemy, etc., and kill sorcerers (1e monks are really effective at slaying magic-users). They are evil, probably opposed to the PCs, but they usually fight a greater evil.
2) The Church of the Law of One, a monotheistic religion of intolerant and stringent fanatics who hate all magic, and (as clerics) gain magical resistance and improved turn undead/demons abilities, but lose spells.
City-states: no more kingdoms (at least human-kingdoms) in this world. More like several city states controlling some land around them:
1. Wisloth: I would use Lankhmar (my old 1e supplement) for this one.
2. Nakhmir: A city of evil, hidden in the mountains. Bleak and black towers without windows, and extensive mazes of corridors and underground chambers below. A lot of slaves (including many dwarves) ruled by a caste of necromancers dealing with demons and afraid of the outside world. A society deep in treachery, scheming, backstabbing, etc. Often sends raids of slavers to capture people, etc. In any case, Nakhmir is the place to sell magical items, learn new spells, etc.
3. Lumina: The "bright city of light" is ruled by the Church of the Law of One (see above). It is not a joyful place, but at least it's mostly devoid of supernatural evil, and relatively safe provided you follow Church edicts and don't practice magic. Lots of crusaders.
4. Asylium: One of the oldest cities in this part of the world. Many advanced technology remains, although the city is another of medieval fantasy. Here, the only allowed cult is that of the God of Knowledge, who incidentally resides in th city. The "god" is in fact an AI in some wondrous machine from a distant antiquity. The "god's" priests are all savants and scientists versed in ancient technomancy. (That would need a new class.)
5. Bastion: Dwarven bunker city, full of warriors and mercenary companies, and owning several advanced technological weapons from the past. Lately these dwarves plan to ally with Lumina and then storm Nakhmir (to release the dwarves slaves held here). Years ago they already sent an army, but it was a disaster as it was wiped out by demons.
6. Gorge Keep: Another fortified city, on top of a mountain, and guarding the highlands from southern invasions. Many haunted battlefields around: danger of undead.
7. Breakwater: Pirate-city, for which I would use Freeport.
8. Zhalygia: Ancient and decadent city of splendor. Live isolated, sustaining themselves with magic of all sorts. City is full of palaces, large roads, etc., but with few inhabitants, and horrors lurking in abandoned places. All the outside world is dangerous wilderness where gnoll tribe roams, etc.
Beyond: Mostly non-human lands. To the south is a dying empire of dragonkin-lizardmen, elsewhere are yuan-ti, mind-flayers, etc. Far to the North, the demon Tsattoghua hold sway in the winterland wastes.
Well, you see the idea.













