I found the following beauty of a quote in a beauty of an article that famed pulp fantasist Fritz Leiber wrote for the first issue of Dragon magazine. Just on the heels of a narrated head-scratching discussion between Fafhrd and the Gray Mouser about our world's wargaming, he launches into what he knows--and doesn't know--about his world setting:
It must always be remembered that I know no more of Nehwon than I have put into my stories [original emphasis]. There are no secret volumes of history, geography, etc., written before the tales themselves were spun. I rely wholly on what Fafhrd and the Mouser have told me, testing them against each other, and sifting out exaggerations and lies when I must. And while my conferences with the Twain have been rewarding, they also have been fewer than I’d wish. I have handled no little books of Ningauble or scrolls or Srith.
For the lands east of the Sea of Monsters, much can, be discovered from the stories written since the map was drawn...The Shadowland, abode of Death and said to contain what some call Nehwon’s death pole, lies east of the Sea of Monsters. Beyond it, still farther east, is the strange land of Eevamarensee, where mankind and domestic animal-kind are alike hairless, but whether this betokens an advanced civilization or decadence only, I know not.
...In "Trapped in the Sea of Stars” the Twain seek to sail to the southern continent(s) and encountering the Great Equatorial Current fail in their attempt, but appear to make some astonishing discoveries about the astronomy of Lankhmar.
...In [The Frost Monstreme] we learn of Rime Isle, a large northern island in the Outer Sea, inhabited by men who appear to be of Fafhrd’s breed at least as to size and situated due west of the Claws and due north of Simorgya (for which see “The Sunken Land” in Swords Against Death).
Punchline: from the time he first wrote about these two characters in the pulp magazine Unknown way back in 1939 until his last published story in the setting, The Knight and Knave of Swords, in 1988, he was making up the details of the world as the two explored it (an illusion of course, but it feels close). If this article is to be believed Nehwon was thus a bottom-up worldbuilding operation.
Personally I have been tugged back and forth between top-down and bottom-up world-building, wildly swinging between an appreciation of the highpoints of the two—something mirrored in the diametrically polar campaign worlds of mine, the eponymous HC (radically bottom-up) and the Domain Game's Nowhere (at least in physical geography and ancient history terms top-down).
At the end of the day, like many others in this corner of the hobby (at least those articulating them in forum and blog posts), no matter I love a setting that has Big Unanswered Questions: large gray areas on the map, unsketched histories,proper names facades—mostly unknown to the creator herself.
For sure those familiar with Leiber's stories in that setting can positively feel the exuberance of discovery that shines out of that incompleteness. Reading through them it's like being in a session of the best of extemporizing GMs, strange new facets pop up at every turn. The reader is exploring Leiber's world as it develops piecemeal.
Apropos to the broader point, but too awesome for me to not quote as a post-script, Leiber's article ends with the two swordsmen pimping Nehwon's mega-dungeons to the D&D audience:
As I regretfully parted from the Twain (somewhere in the caverns of Ningauble, of course, for they’re the only place I know of where Nehwon and other worlds link — see “Adept’s Gambit” in Swords in the Mist) Fafhrd remarked, “Don’t forget Stardock when you write for these wargamers—a whole vast Dungeon inside Nehwon’s mighiest mountain, with routes both on the mountain and inside it.”
“Better yet Quarmall, and not half as chilly,” the Mouser in eagerly. “A vasty [sic] underground world of many levels, a nation in the mines! There’s a Dungeon would send wargamers ape!”