I've come across the expression "mega dungeon" quite a few times on the net. It seems like it's not just about the size of a den of monsters but it might imply a play style or something. What are the specific traits of a mega dungeon?
I believe that a megadungeon is one in which the campaign is intended to focus around. So it is not just a huge dungeon, but one where the game's buy-in is the idea that the players will be traversing the dungeon for pretty much the entirety of their careers.
Otherwise I don't think there are many defining features. Some megadungeons seem to try to present an entire plot or some type of ecology or reason behind its existence (such as Shackled City), while others are so big and expansive that almost every floor has its own distinct story, and is really only connected through the plots players get involved in (Stonehell Dungeon, from what I've read, and somewhat Castle Whiterock).
- The dungeon has multiple, interconnected levels or zones
- The further you go the more dangerous it gets
- An entire campaign can take place in and around the mega-dungeon
Past mega-dungeons include World's Largest Dungeon, Monte Cook's Dungeon a Day, and Dungeon Crawl Classics #50 (the boxed set).
One aspect of a mega-dungeon is the lack of overriding, campaign driving plot. A mega-dungeon just "is", has been there long before the party, and will continue to be there long after the party is gone (at least thats the vibe the DM should give). No one party could ever map it, even if they devoted all their time to it, and it should never have a linear "end" or a big bad ruling the entire thing. Basically, it's "bigger" than any one little group of adventurers, and it should be played that way.
A mega=dungeon is NOT just a giant dungeon, or a large underground campaign area. It should dominate the area, to the point of having towns built around/nearby it, and legend written about it. It should have a history, perhaps not known from the beginning, but gradually pieced together as the group investigates the twists and turns of the underdark corridors.
Probably the best published example of a mega-dungeon is Ruins of Undermountain. Large, sprawling dungeon levels with no over riding theme or reason except for exploration, with dozens of plots, monsters, power groups, and mysterious areas. Also recommended are Stonehell Dungeon, Rappan Atthuk, and Castle of the Mad Archmage.
To add what Ry St said the term originated to describe the revival of the style of the earliest campaigns that revolved around the Blackmoor Dungeons (Arneson), Greyhawk Dungeons (Gygax), and El Raja Key (Kuntz). Like Ry St describes they were large interconnected levels that got tougher the deeper you went.
Ironically if you read the stories of the time, the campaigns were just as much about establishing a barony in the wilderness as they were dungeon crawling. (typically happened later in the campaign). Then slightly later the focus shifted back as all the newer players wanted to do was go dungeon crawling.
An additional note is that these early dungeons were very sparse compared to the published modules of later generations. In First Fantasy Campaign (Blackmoor) and the few things we seen of Greyhawk and El Raja Key maybe only a 1/3 of the room had something in them. Usually described in one or two lines. There was usually one, two, or three detailed encounter rooms/areas per level. Then there were wandering monsters.
Simply, a huge dungeon.
That's an astute question, though, as a megadungeon does imply a 'sandbox' (non-linear) style of play. The players typically have a wide choice in choosing a path, and may usually backtrack to a Known point to reorient. Most megadungeons are suitable for placement within range of a civilized area. Some actually involve little or none of the typical 'return to town' breaks for resupply, instead including safe zones for rest... and even shops!
The megadungeon is a huge environment often lacking an overview, e.g. a 'map of the realm' found in an overland campaign game. Shorter line-of-sight can decrease reaction time. But constrasting sharply with the 'anything goes' nature of wilderness adventuring, the dungeon environment is somewhat safer in that regard due to those same physical constraints.
Although the nature of a megadungeon often strains credibility, it is common in many 'home games'. Environmental complexity is usually required for outdoor map planning, and so novice game masters commonly use this simpler mode as one of their first major design projects.
The dungeon is large enough to be a campaign setting in its own right.
That's the real key. If you are not camping inside the dungeon, and even leveling up inside the dungeon, then it's not a mega-dungeon.
AD&D Forgotten Realm's Menzoberanzen is one of the earlier ones.
I define a Mega Dungeon as a space full of rooms, corridors, monsters, traps, and challenges that is bigger than all the rest. How big does it need to be to qualify? Well, to me, if you can explore the whole thing in a week, it is NOT a mega dungeon! Famously, a mega dungeon usually has more than one level, probably at least six. Each level needs a bare minimum of 25 rooms. The total dungeon should have at least 200 rooms to explore. And of course, the whole place should be full of dangerous creatures.