Yesterday I pulled out some graph paper and stared sketching out what I thought would be the first level of the dungeon. I had spent some time looking at maps other had posted, at random map generators, and put pencil to paper.
And I wasn't happy.
I enjoyed it, and I think I came up with an interesting dungeon, but it was just... too random. I wanted there to be more story behind the dungeon, not just... a crazy hole in the ground.
So while cooking dinner I thought about what this megadungeon was built for. I decided that it was originally built for the reason I imagine most dungeons are built for, storage under a castle or other fortress. Now what sort of things would a castle store? Thinking about what sort of things I store in my basement I realized a castle would probably be just like that, only more so. There is food storage of course, but also stuff storage. Imagine after a couple of hundred years how much junk might accumulate in someones huge basement. Things like seasonal decorations, old tools, broken items of furniture, grandma's plates and bowls with the really ugly pattern, the stinky stuffed owlbear wearing wizard robes and holding a decorative wand, and even more would be kept down in the basement. There are probably treasures mixed in somewhere, or at least there were before mold or monster (or monstrous mold) got to them.
The first level of the dungeon would be for bulk storage. Things that would be too large or too heavy to want to move too far from the main castle. The next level would most likely include smaller stuff, and prisoners. You don't want those screams echoing up into the castle now do you? There are probably areas where the garrison can fall back to should the castle be taken, and crypts. The crypts can be pretty extensive, extending down and out for quite a ways. Further below that are the vaults, and the most heavily trapped area of the dungeon.
This is how the dungeon was originally built, many many years before. 150 years before now the castle fell, and the area became wild as civilization was driven out. The ruins of the castle were raided, and the dungeon plundered and inhabited. Over the last 150 year it's inhabitants have expanded and remodeled the dungeon. Recently a trade route reopened nearby, and stories of the fallen castle and it's vaults have begun to spread.
So after dinner out came a fresh sheet of paper, and I basically drew large warehouse style rooms, pretty boring by themselves, but I intend to fill them with both the stuff from the castle (what's left anyway) and a humanoid clan and a human bandit gang that has set up shot to raid the trade road. I figure the bandits and the humanoids have come to some understanding about the only exit to the surface from this level and they keep themselves segregated. Members of both the goblin band and the human gang will be out of the dungeon and will return after the heroes have passed through the first time, or while the heroes are in there if they decide to camp out in the dungeon. There is a hole down to the 3rd or 4th level (I haven't decided), and stairs to the 2nd level.
The second level will have some undead, as well as critters, and more humanoids. From the second level there will be access to the first level, and the 3rd level via stairs. There will also be access to the 3rd level via holes dug in an attempt to avoid the traps and get into the vaults. Sections of the 2nd level will be cut off due to collapsed corridors. These sections may or may not be accessible from the 3rd level. The crypt section of the dungeon may be accessible from the surface. Might be blocked off though.
The third level will have lots of undead, as many have died trying to get into the vaults. I also figure that a wizards workshop will be down here, which can include all sorts of fun things. The wizards workshop can delve deep into weirder sections. There will be further additions to the dungeon on this level by kobolds or other diggers. The levels below the 3rd will include natural caverns which will connect with some dwarven/kobold mines. There will also be a hidden escape route from the 3rd level to an as yet undiscovered location on the surface.