The original Shackled City campaign that we ran was in a Dungeons and Dragons 3.5 edition format and some of the things that may be in here will be pulled from that set of rules. Our group, however, has moved on to Pathfinder and going forward I will be developing this campaign in that format.
Occipitus itself is one of the planes of the Abyss. It is loosely tied in with the ongoing campaign of the group that I game with the most, the Wyrmfang Chronicles. The Wyrmfang Chronicles occurs in a modified Forgetten Realms format. The rules we have used for the Wyrmfang Chronicles include Advanced Dungeon & Dragons, 2nd Edition. We moved on to 3rd edition briefly, then 3.5e for a time. Now we use the Pathfinder ruleset.
For the Shackled City campaign that was the basis for my realm of Occipitus, the City of Cauldron was located in the southern reaches of Faerun.