So, what ought to be in a dungeon?

Some combination of traps, treasures, monsters, and secrets.

Why is the dungeon there?

This question is more important for the GM to answer than it is for the players to uncover — and some GMs might never need to answer it at all.

I eventually figured that the treasure content of a dungeon should be predicated on the encounter content. Treasure isn’t worth much of anything if you don’t have to work for it. The “negative space” will also be based on encounters.

If 60% of all encounter spaces are “blank” encounters, and you start with 10 encounters, then you should have about 15 blank encounters. Does this mean your average dungeon will have twenty-five rooms? I don’t see why not.

What we’re really concerned about is what appears in those 25 rooms.

It means you need 25 rooms’ worth of set dressing.

That seems to me like a zoning issue. You need to divide the space in the dungeon into parts, and determine how each of those parts is used.

So you need to define the dungeon’s function.

Link: Classes of Dungeons
(2014 October)

This is only part of the puzzle though, you’ll need to know why the dungeon is there, and for that you need the dungeon’s origin, “how” it got there.

So, what is a dungeon?

Why is it there?

Going all the way back in literature, the adventures of ancient heroes took them up mountains and deep into caves. When they went up, they met with the gods. When they went down, they entered the underworld.

Dungeons are borders. They’re portals. Thresholds. Between-places.

A dungeon is an intersection of two or more worlds.

This is incredibly evident in a game like Diablo, where you explore the dungeon beneath Tristram until you find yourself in Actual Hell.

Not all dungeons need actually connect the two realms, but they need to get close enough that you can get a sense of something otherworldly.

If you’re looking, I mean.

We can have as many dungeons as our cosmology has worlds.

If you’re using 3e’s Great Wheel, you can have like, 16+ dungeon types. If instead you’re using 4e’s Points of Light, you have like, 5.

Don’t underestimate those five though, they’re bound to be more compelling than the eight or so “Incrementally Evil” Outer Planes of 3e.

The more you know.