Hussar
Legend
/snip
But what is a dungeon. In another thread, Doug talked about degrees of abstraction. If you pull back a degree or two, a city becomes a dungeon: streets and buildings are analogous to hallways and chambers and individuals, groups and political forces become monsters, traps and tricks. Could you pull out another degree and create a "dungeon nation", a land so embroiled in conflict that roads become hallways and villages become rooms?
I don't want to get too far away from the core discussion here, though, which is: how do we classify dungeons based on their function in play? Can we? Are there any hard lines or are we likely to find most dungeons fall in the spaces between whatever categories we decide upon. And if so, is there a point in categorizing them at all?
On the first - could a nation be considered a "dungeon", I'd say certainly. A dungeon, at least from one perspective, is simply an adventure flow chart. You could apply that same flow chart at almost any scale, from a small four room dungeon to an entire world. I would go so far as to say that some definitions of "sand boxing" would create a world which looks very much like an adventure flowchart or dungeon. Go to point A, encounter X, from A you can go to B, C or D and have different encounters. The greater your ability to travel (going from horses to flight to teleport) simply changes the connections between fixed points on the flowchart.
As far as categorization goes, it's like any categorization in that you will never cover every single eventuality. It's like genre, all you can do is hit the generalities and realize that there is a fair degree of overlap between different types.
World's Largest Dungeon, for example, could easily be a Mega-Dungeon. There are more than a few areas where you can add to the maps if you like, and, considering how freaking huge this place actually is, I think it qualifies as pretty much endless. You can easily have several parties taking different paths and never seeing the same area twice.
OTOH, it could also be played as a Campaign Exploratory, or even and Adventure Path Dungeon, to use Lanefan's excellent definitions. It all depends on how the DM sets it up and wants to use it.
I don't think you can definitively pigeon-hole many products, simply because the DM has so much input into how it's being used.