In yesterday's post about the megadungeon possibilities missed in Planescape's hub city, Sigil, Michael Moscrip asked very nice and polite like for a setting supplement. Since he's ill-advisedly talked about running a Dickensian-like Sigil adventure—even going as far as to create a portal to it from the Google Plus pocket universe's Castle Nicodemus–it's in my enlightened self-interest to help out.
I am way too swamped to do a great sweeping treatment, but here's a stab at a few things to get someone going on creating a Sigil underworld of their own.
First of all, we have to deal with John's good, tough question from yesterday:
“My trouble with the Catacombs is that, in a way, they're too big. On the one hand I like the idea of the entire torus being riddled with dungeon passages. But if I have one entrance in the Twelve Factols and another entrance in the skull library, ten miles away, how can I ever link them up in a way that matters? For all practical purposes they're two separate dungeons. Do I "cheat" with miles and miles of empty passageways? Weird dungeon spatial warping?”
Yes to both seems most appropriate given what we know, namely that most of the undercity was created as work and living space by the tinkering dabus and that as many inter-dimensional gates exist below as they do above. If this section is to be believed you have something as reality-defying as the Mythic Underworld:
“Nobody's ever dug so deep that they come to the 'other side' of Sigil's toroidal surface. Those who dig deep enough simply disappear, presumably stumbling into a random plane, and nobody's ever been able to figure out a pattern to it.”
So how do you run such a sprawling beast? I suggested trying to apply my pointcrawl system (John answers his own question with some interesting remarks about a hex-based solution in the comments).
Above is a pointcrawl map listing every site that gets a mention in canonical material. Underscoring yesterday's point all but two of the sites, the Gurincraag (a dwarven neighborhood under the Lower Ward) and the Twelve Factols Inn (mentioned yesterday), are drawn from Torment.
Large boxes indicate major sites, worthy of a single dungeon level or two. Green boxes indicate a site that has humans or other overworld living creatures living there, blue indicate wholly “monster”-inhabited levels. The smaller uncolored boxes indicate “sublevels”, special areas that would typically be 2-10 rooms in a normal dungeon.
Unbroken lines indicate the more normal walkable tunnels and other . Squiggly lines indicate places joined by a “teleporting” portal (all such portals have some kind of material key to them). Arrows indicate that the connection leads downward in that direction. Connections should be of variable length and indicated on the map (out of laziness I didn't).
What about stocking these places? Here's where I am going to punt. Since the vast majority of this space is treated in the computer game—many of these areas get a highly detailed treatment in the numerous online walkthroughs of the game. You will find a number of useful links to maps and descriptions on the various levels and sub-levels here at Sorcerers.net.
Eyeballing these maps they feel too cramped, too square and oddish in shape to be a perfect fit for tabletop maps—and you have to deal with the meta-gamey knowledge of those who have played through—I would strongly advise using them as a launching pad, expanding the maps and areas over longer dimensions.
End of backseat driving, now go and build.