We have heard the names before: Castle Greyhawk, Undermountain, Night Below, Moria, Rappan Athuk, Castle Whiterock, The World's Largest Dungeon. The megadungeons. These are the engines that powered countless the Old School campaigns. Recently they have been called the tentpoles of the old school campaign and with good reason. But where is that crossing point between dungeon and megadungeon? Is it like that old definition of pornography- I know it when I see it?
Accross the old school blogs we have seen a lot of talk about megadungeons and even seen examples of new megadungeons appear. But what defines a megadungeon? Is it based on design principles? Capacity for character advancement? Is it a publishing threshold like a page or word count?
Melan on Dragonsfoot has stated that a megadungeon is "A dungeon expansive enough to accommodate an entire campaign of substantial length." At first glance this seems a throwaway definition. However as you unpack it, we begin to see that a dungeon fitting this definition must include a lot. Most dungeons do not survive to a second visit by the party. And most recorded megadungeons were not visitied multiple times by one party but multiple times by multiple parties. The key words here are ENTIRE, SUBSTANTIAL, LENGTH. The megadungeon must be multi faceted and different from part to part to keep a whole campaign going. This need to keep it interesting dictates that the "gaming terrain" of the megadungeon vary, sometimes wildly.
In this respect the megadungeon can have several functions. It is obviously the principle adventure location for a campaign. But it can also serve as an interface to the whole of the DM's creation. Portals and other means of transport allow a megadungeon to act as a sort of subway to other adventure locations. We see this with the demiplanes of Castle Greyhawk, and the portals undoutedly found in Undermountain. This allows the entirity of the campaign to tie back to the megadungeon. Not just in that the megadungeon is location 0 but rather that all of the gaming world is concievibly accessible through it.
While a campaign is a series of linked sessions and can be very short, the megadungeon in a DMs world can provide a constant and help it to persist from party to party and gaming group to gaming group. In this respect a megadungeon would probably have to be planar in nature and essentially limitless. I believe this is why Castle Greyhawk only came close to seeing the light of day as a summation of the spirit of the original. The original was too expansive. This reveals a seldom realised corollary to Melan's definition: The extent of a megadungeon is a function of the number of players, parties, groups and characters that have visited it. That is it grows by use. The megadungeon is a megadungeon only if the campaign is extensive enough to warrant its continued and constant growth.
Certainly you can create a megadungeon whole and complete as Athena from the get go. We have seen it done in a lot of products. But in the true megadungeon development is always just several steps ahead of the players, and those places that have been visited can be visited again. But curiously if you carry this out to its logical conclusion you realise that the megadungeon is really just the sandcastle at the center of the sandbox as that really is what a megadungeon is: The sandbox concept applied to the dungeon rather than the world.