These are some parts of a megadungeon I'm working on...
(I realize the pic is blurry and backwards, but that keeps my players from figuring out what's going on in there.)
Here are some pictures I like today...
Down here is a picture I took while working on one of the maps for the upcoming book...
And here is a picture of me eating a frog-shaped cupcake:
Megadungeon Key Mechanic:
How many times has this happened to you:
DM: "The door is locked."
Player: "Oooh, try that key we found 4 days ago!"
Player 2: "Yeah, I try that key."
DM: "Ummmm...which key?"
Player: "You said it was an 'ordinary-looking brass key' and found it in the north turret."
DM: (fuck) "Uhhhhh...surrrre, it fits."
Let's face it, there's a lot of locked doors in your megadungeon, and there's a lot of keys, too, and most of the time, which key matches which door isn't really the point. What's important is just that the key be far away from the door it's supposed to be unlocking so that the locked door seems mysterious for a while.
Like if the door's on level 5B behind the tapestry of the goat farm, the key is on level 9c in the belly of the spherewhale. But who has time to keep track of all that?
Here's what you do: when the PCs find a key, tell them they've found (roll %ile) "Key 37".
Now what they think is that you have at least 37 keys. Which should freak them out.
However, what it really means is that it has a 37% chance of opening any (or almost any) locked door in the dungeon they try it on (roll secretly). However, the first time the key works, that's it, it's no longer useful. It was the key for that door.
Assign a high number if it's probably just some dead monster's "house keys", assign a low number for that "Holy fuck, that key we found waaaay back on level 2 opens the Temple of 9000 Terrors!" effect.
This way you can put keys in with monster treasure without worrying what they open and you can have unpickably locked doors without worrying where the keys are gonna come from.
Can't open a door? Kill a few more bugbears and you might find the key...