Then I actually, finally, read the thing (I posted some comments in the Reviews section on Dragonsfoot). Here is what the module has to say about Greyhawk (emphasis mine):
As I read on, it became pretty clear to me that Mystara remains the default setting for the Keep. (Just look at the elaborate changes required to actually adapt the module to the World of Greyhawk). The module was published in 1999, a few years after Mystara was officially transported into the 2e sphere (with the K:KoA and G:KoM boxed sets, etc). Furthermore, it requires virtually no "adaptations" for Mystara.If you're using the World of Greyhawk setting, the Keep should be located in the southwesternmost part of the Yeomanry, a lordless land of freehold farmers shielded by monster-haunted mountains from the great desert beyond.
Here is a list of Mystara references in the module:
1) Dubricus d'Ambreville is a first-level mage, "scion of a famous family of wizards", who came to the Keep several months ago seeking an adventuring party to join up with.
2) "Third", warrior maid of "Maruda" (sic) "hails from the distant underground city of Cynidicea" from the desert across the mountains.
(She apparently doesn't speak Common, but if Cynidicean is related to an older dialect of Thyatian, then PCs may be permitted to roll an Int check to understand her. In fact, "Maruda" might simply be a clumsy mispronounciation of "Madarua")
3) Tomas and Holga are a couple of strangers who turn up at the Keep. True, they originally hail from the distant Lendore Isles, but are en route to distant Cathos City (another island).
4) Another reference to "Maze of the Riddling Minotaur" is a tapestry of "Lady Durnsay and the Bugbears." Lady Durnsay (of Cathos) is described as a famous adventurer of four hundred years ago.
Compared to that, the only Greyhawk references are the various deities, such as Erishkegal and Nergal, which could easily have Mystaran counterparts (like Orcus).
There's also the matter of Quasqueton (which still leaves me scratching my head, a bit) but that's easily explained away as a hoax (since even the entrance to the dungeon is not as described in B1).
Heck, the only thing required is a reasonable backstory for the original founder of the Keep, Macsen Wledig. Since there was a historical figure with this name from Roman times, a Thyatian connection is not hard at all.
(In fact, I would have that Macsen, or Lord "Maximus" originally hailed from Caerdwicca on the Isle of Dawn. He could have received a landed lordship from Duke Stephan in the 980s and built the Keep within a few years. According to the module, after twenty years or so "the disaster of war overtook the land" and Macsen marched away in the defence of his distant homeland, never to be seen again. This could have been the conflict detailed in "Wrath of the Immortals" and Karameikos becomes a kingdom not long afterwards. K:KoA is set in 1012 AC and so "Return to the Keep on the Borderlands" could take place sometime after that. Both products use the 2nd edition AD&D rules.)
It always kind of bothered me that the Keep was kidnapped and transplanted in the Yeomanry, but after reading the module cover to cover, I think the author must have actually started out with Mystara as the default setting, and the suggested location for those using the World of Greyhawk was just added later.