I ran across a megadungeon article from last July. I know, I am a little behind, as I was dealing with my cancer when it came out.
In the blog, DM David lists 5 reasons why megadungeons don't work for most players. I have replicated his list here:
1. Players never saw any examples.
2. The ecology and rational of megadungeons seemed ridiculous.
3. Play styles expanded.
4. Megadungons can feel monotonous.
5. Computers do megadungeons better.
Now, I think all of his premises are wrong. Let's start at the beginning:
1. Players never saw any examples.
A few examples were printed through Judges Guild early. Bill Webb and others contend that Tegel Manor is the first printed megadungeon. I lean towards Jennel Jaquays' Caverns of Thracia. There are other examples that people cite too (City-State of the Invincible Overlord's dungeons, the First Fantasy Campaign and probably others too).
The earliest TSR megadungeons didn't come out until the early-mid eighties: Temple of Elemental Evil, and the Lost City (B4).
If you argue that Greyhawk Ruins (first published in 1990) is the first true megadungeon, you could make David's point, but I have listed several examples from the first ten years of D&D that qualify in my expert opinion.
2. The ecology and rational of megadungeons seemed ridiculous.
If any of David's arguments hold weight, it is this one. But, holding to good standards of megadungeon design removes the gravitas. If only 1/3 of the rooms in a dungeon hold combatable creatures, that leaves 2/3 of the space to hold food/resources/restrooms etc.
In Mord Mar, nearly every dwelling had a mushroom cellar. Rats run amok through the refuse and detritus strewn about by humanoids. They make tasty snacks for goblins. Bats find ways in and out of subterranean structures all the time.
Rivers dot my megadungeons and when it rains, that water often drips through several levels. It doesn't take much creativity to add a room with a lot of buckets and some dripping water. (We will come back to this idea in #4).
3. Play styles expanded.
I really hate David's exposition on this one. Early D&D was filled with adventures and stories. It wasn't all just "stab the monster and take its stuff." The players used intrigue on each other. They had motivations and ideals and wanted more than David infers.
I don't think play styles expanded at all. I think most people don't know how to craft a story within a megadungeon. The DM can't handle the possibility that the players may side with orcs, goblins, gnolls or attempt to eradicate all three.
4. Megadungons can feel monotonous.
Sure, megadungeons can feel monotonous. So can a forest, or a jungle or the miles and miles of desert sand. Any locale will feel boring and stale if the DM can't create excitement in their tone and their words.
Have your characters ever stood beside an orc raiding party attempting to take control of the water room as the rains started to fall? Maybe they are the diplomatic type and hammer out a solution that each group has a representative doling out water to their compatriots. Maybe the rapscallions seized control of the room and are auctioning water to the highest bidder? Putting good plot hooks in an adventure makes it feel dynamic. Don't blame bad DMing on the megadungeon, 'k?
5. Computers do megadungeons better.
Computers don't do any RPG better, full stop. The computer has certain parameters that need to be followed. That by definition makes them closed-loop games. Unlike D&D and megadungeons, where possibilities are endless.
And David's concluding line:
"Although good design can yield a megadungeon that proves fun to play, ordinary dungeons can bring the same advantages. Today’s gamers tend to create megadungeons to foster nostalgia or to enable episodic play."
I don't think a smaller dungeon can bring the advantages I look for in a megadungeon. Out of the three tentposts of RPGs (role-play, combat, exploration), exploration is by far my favorite. Sure, I can also explore a forest or a jungle (both of which become repetitive and monotonous quickly) but a megadungeon adds so much more. It adds factions, and hidden rooms and secrets and a history to unearth.
When you play a well crafted megadungeon, you peel back the layers like an onion, learning new things about the environment. And yes, I also like the episodic play aspect, as I have literally hundreds of friends that play D&D and are all welcome at my table.
