First of all, I think the prefix "mega" is unnecessary and has done harm to the artform of what I'll call instead a tentpole dungeon. It conjures images of 600 page tomes with long room keys that are off-putting to read and play. What we call megadungeons used to just simply be referred to as "dungeons", a setting for a campaign which, due to its ever-changing and expanding nature, holds capacity for infinite play.
Making one of these is much easier than it sounds, and is not too distinct from the kind of worldbuilding most GMs do now. Instead of putting everything above ground, put it underground into a dungeon. If you have a city, make it take up 3 big dungeon rooms instead. Draw 5 rooms, give it a theme, and you've got a quarter of a level or a sublevel. Making and running a megadungeon is easy.
I don't know where I read this but there's an observation that the term "Dungeon Master" exists because, in the early days of the hobby, it meant what it says on the tin: Gygax had Castle Gygax, Arneson had Blackmoor, every Dungeon Master had a unique, personal dungeon that was theirs, that they were master of. To be a Dungeon Master is to create your dungeon.
This isn't much of a post by me about megadungeons, though, as it is a curation of my favourite OSR content that addresses megadungeons.
The first and most important is the first three pages of 0e vol 3 (the rest of vol 3 as well as the DM sections in B/X are also indispensable though).
Crawling through a megadungeon is like crawling through the Referee's mind.
If you run a megadungeon campaign like an adventure path, then it immedi-ately becomes a tedious slog of combat after combat. If you try to run it like asandbox, the structure of the megadungeon itself works against you. Not onlycan you not see the other areas of the sandbox, most other actors within thedungeon have plans who's scope likely excludes the characters. Who careswhat happens into the depths, when they are trapped in the mythic under-world? Megadungeons are not designed to facilitate player driven goals thatare necessary for a sandbox to function.
There are elements of strong game structure in megadungeons, particularlyrevolving around encumbrance, time and light, movement and vision. Thesedon't make any sense in adventure paths and are frequently less useful insandbox games.
These are important because they provide weight to the idea of the Megadun-geon as an inimical place. If you go 120' forward, You've caused a hazard dieroll and resources available have decreased. Every step has a cost, and tryingto get something—anything!— of value out this place is hard, because it pulls onyou, weighing you down, refusing to let you leave.
It makes it mean something to the players. Territory explored is not onlyrevealing the map; it's gained knowledge, that allows you to descend deeper inthe depths of the mythic unknown. It is compiling this knowledge that empowersthe player to engage in every more risky challenges in the depths.
MEGADUNGEON MEANING AND ROLL(E)
Megadungeons are mostly empty, because they are a stage.
And us, the players.
It must serve three functions. It must obstruct and confuse characters in a waythat challenges the player, it must be mostly empty so it can hold the emergentdrama between players and dungeon actors while exploring, and it mustcontain treasure at intervals to provoke a reward response in players.
What happens is that while the players explore, they quickly become aware ofother groups of monsters or players that are moving through the same dungeonarea as they are. Most are not immediately hostile, but everyone in thedungeon is an opportunist. Fights against equally powerful non-player charac-ter parties are often fatal, but after they've fought a manticore, it might be adifferent story. It's likely they think the same about you. These relationships andrivalries persist from session to session. It is a sea filled with pirates and sharks.And since the door only opens once each week, you're stuck dealing with whoyou run into this session, while you're trying to accomplish your goal, meaningthings usually go one way. . . or the other.
Sometimes, there are dragons.
Have you ever been hunted as a mouse? If your character survives to tell thetale, it will be memorable. If they could slay such a beast? Unforgettable.So, no. Not like a sandbox or adventure path. Yes, more focused on someunusual rules. More like an emergent adventure that challenges the playerthemselves. A fun game to play with a rotating group of friends. Friendly rivalry.Sort of like a party game with dares. You know—a Megadungeon!