How should I go about fleshing out a megadungeon?

[Ember1]

I've been running the Carrion Crown AP online for a few months now, and my players have expressed a desire to continue after the campaign by going through Gallowspire. I've since picked up the Dungeons of Golarion book to get some inspiration for it. I've had a few ideas about things to add, but I'm completely lost on where to begin. I have looked at everything I could think of, (Rule of Fear, Tyrant's Grasp AP, Dungeons of Golarion) but still am stuck on even really starting it.

[Bjørn Røyrvik]

Describing any structure starts with two things: who built it and why?

The latter will inform function (form follows function) while the former will inform details (size, shape, decoration, etc.).

What sort of rooms does the actual spire have? What does its 1st floor look like?
The tower was created by, or at least redecorated by, TB so it probably lacks things like toilets, kitchens and larders, but might have slave pens for those to be forcibly converted, or feed those undead that need to feast on the living. Probably some libraries, long looted, and labs, long destroyed. Maybe a scrap or two of ancient writing is left, like 'note to self: demote Bob', a letter or two from persons unknown (like one from Factol Skall telling TB that he's Doing It Wrong).
What does the entry part look like? Was it designed as a reception area for visitors or more like a gathering area for TB's elite servants before venturing forth?
Are there permanent rooms for anyone but TB? Guest rooms? Is there nothing but labs, libraries, storerooms, war rooms, etc.?

As for inhabitants, pretty much any and all undead are appropriate. Feel free to throw in a new kind or two so the players have no idea what to expect. As many or as few as you wish. These will be the ones TB had with him when he was trapped and he hasn't had opportunity to create more so what he has are as ancient as he.

Lastly, find other mega-dungeons and see what they did. Steal ideas ruthlessly.
Classics like Undermountain, or newer stuff like Rappan Athuk, Purple Mountain or Kostakep.

[PFRPGrognard]

Just grab a published mega-dungeon and change the names to match your campaign. All-Stars Take on the Mega-dungeon, Castle of the Mad Archmage, Rappan Athuk, and the World's Largest Dungeon are a few non-Golarion dungeons that could easily be converted.

If you just want the Paizo material for Gallowspire, then buy all of the existing material and make your own. That's how you flesh it out. You just do it.

[Andostre]

Quote:

How should I go about fleshing out a megadungeon?

The spell stone to flesh.

[SheepishEidolon]

It's a big project, and big projects need to be divided in steps, to avoid getting lost. In addition to the previous advice, I propose the following systematic way:

Step 1: Goals

Figure out what "go through Gallowspire" actually means to you and your players. Note your associations first, then ask your players, starting with the quietest (so they can't just nod to a louder player).

Step 2: Approaches

Sort the associations and treat them as design goals. What are the approaches for each goal? For example if two players want "powerful undead", you could note "mean special abilities", "spell-casting" and "really big and hard-hitting" as approaches. If someone wants "horror", figure out what horror actually means (Wikipedia, Paizo's Horror Adventures etc.), only then move on to approaches.

Step 3: Material

Check the existing material for inspiration. Scan it for implementations to your approaches. For example, if there is a nice stat block for a lich, that could be one implementation for your "spell-casting" "powerful undead". Maybe the existing material will inspire you to add further goals and approaches - be careful not to overdo it, especially with goals.

Step 4: Design

Sketch / print a map, assign an intended PC level and add encounters to it. Keep it abstract, you don't need any stat blocks yet. For example the door section could have "2 undead gargoyles, a skull pile". Add markers for difficult encounters. Look at the map and add and remove things until you are content. Check whether the sum of XP and loot value is roughly right. 30% more or less than originally intended are no biggie.

Step 5: Implementation

Add images, text and stat blocks. Steal as much as you can and modify where needed. If you find yourself spending multiple hours on one thing, consider a different way, at least for the next time of such a thing.