I made a 120 Room Dungeon in a Month

Originally published May 13th 2025

So I run an open table D&D game for a youth center. I got myself a notion that I had two months before I was gonna be running a game so I might as well try doing some daily dungeon keying and by the time I had to run a game then I’d have something sizable to run for the folks at the youth center, an d I could keep expanding it until I had a full megadungeon to one day release.

I started brainstorming and decided on a general outline of the dungeon I came to this general premise

A week later I was working on filling it out. I decided I was gonna do things a little differently, I was gonna work from the bottom up, so foreshadowing on earlier floors was easier, and I was going to follow the advice of Sean McCoy and write rooms in pairs . With that I set out with my roughly 2 months hoping to do 2 rooms a day.

However astute readers may notice I said I made a 120 room dungeon in a month, not two, so what happened?

Well mainly the youth center wanted me back sooner than I originally thought and as such I had half the time, I learned this two weeks before the game was to start back up so I had to kick it into high gear and double the rate of key fills per day. This increased speed made me realize a few flaws with my methodology I want to talk about.

Flaw 1: The Initial Premise

I like the ideas I layed out for each layer, the strong theming was inspired by the videogame The Binding of Isaac where each floor has a very different feel to it. I think this is solid in principle but I now realize that this dungeon idea didn’t really have a backbone. There were implied layers of history, but none of the stuff you need to help really direct the keying.

If I was in the position to start from scratch I’d think of a strong cultural history for this place and then add it more but the way it sits I’m half way through the first draft so I just know in second and third drafts of this I am just gonna need to write some history and change a few keys to reflect this.

Also floor 2 is incredibly weak, I knew it going into it but by the time I reached it I realized just how anemic the idea is, vibes can help a lot but idk generic vibes doesn’t really mean anything. Floor 3 being the one with all the undead made me hesitant to put undead in floor 2 so that was also a limit on the palette of something that probably needed more flavor to it.

Flaw 2: Bottom Up

Is starting from the bottom and working your way up necessarily a bad idea? No not at all I really recommend it if you have the time but fundamentally because I was on far more of a crunch than I initially anticipated it means that I was sitting in a situation where I had 70 rooms done and I was freaking out because I didn’t have anything ready for the table. Knowing what I know now I think I would have certainly started on floor 2 and worked my way up instead of floor 3.

Flaw 3: More Than Keys

A dungeon is more than just keys, it’s also a map and random encounters. By the time I had 120 been 31 days (well more actually because in the beginning I got excited and did a little of floor 4). I still didn’t have nearly enough to run the thing. I hadn’t drawn up the maps for any of the floors and had to rush to get them done. Also I needed encounter tables, now I could have simply used the ones found in a myriad set of games for just running at a youth center.

However I don’t really like using generic monsters at all, and the floors I had were so heavily themed I would have regardless had to rework any tables there. Designing monsters is also a lot of work and getting 15 unique monsters per floor like I originally planned and still plan to do is a lot of work.

It was just more and more confounding layers of work on an already incredibly tight timeframe for something of this size.

Here are a few more things I did wrong, like not really using a dungeon fill procedure for days where I couldn’t come up with ideas at first, but that’s far more minor. The Sean McCoy advice lead to some really good rooms though so that was nice. There aren’t a lot of grand lessons to be learned here but I hope someone reads this postmortem (does it count as a post mortem if I know I am only half done?) is useful for someone else considering working on a megadungeon. Don’t attempt to do 4 months of dungeon23 in 1 lol.