I'd like to use it in a campaign, but I don't know where the entrances to this place are located in Sigl or on the Prime. Help, please?
Where are entrances to the World Serpent Inn?
That's what I've gathered too.
Is the World Serpent Inn is supposed to replace the Astral plane, or to serve as an additional transitive plane?
"La la la, I'm a girl, I'm a pretty little girl!"
--Bel the Pit Fiend, Lord of the First (in a quiet hour of privacy)
It's a demi-plane which has an entrance that jumps to different locations frequently, so it's not meant to replace the astral.
I read up on the place for its inclusion in the Venues list and I couldn't find anything that suggested a specific location for its portal (or portals) in Sigil, though it is certainly accessible from the City of Doors. There's also no clear indication of whether or not it has more than a single access point in Sigil. Consequently, in the database I listed its Ward location as "any".
Also, if I recall correctly, there's nothing in the place's recent Dragon magazine writeup that would suggest it's portals change location; only that there are many such portals. For instance, there's a static World Serpent portal located in the city of Arabel in the Forgotten Realms setting. Please correct me if I'm wrong.
From the Manual of the Planes web enhancement:
Now, that said, there are places on the Material Plane that reliably lead to the World Serpent Inn. The Wild Goose Inn in Arabel (in the Forgotten Realms) is one of these. The Wild Goose Inn in Greyhawk City and Helkam's Pit in Irongate (both on Oerth) are two more. Dragon #351 mentions others in Ecstasy (in the Outlands), in Zakhara, Kara-Tur, Krynn, and the Demiplane of Dread.
There are TWO Wild Goose Inns?!? (Could they somehow be the same place???) Also, does Mitchifer have a last name or alignment (or any other stray details...)?
They change frequently, which is how the place gets a variety of business. I could be mistaken about that though.