Naturally, having yielded my responsibilities as DM, I now miss it. One of things I wish I had done was construct the campaign around a mega-dungeon, rather than use episodic pre-generated adventures. Part of my decision to use pre-writ adventures was because I was frankly unsure whether a group could develop out of this. So I didn't want to commit a ton of time to a campaign only to see it fizzle, and I instead focused on painting miniatures that will never see a tabletop. Sometimes my internal "logic" amazes me.
Megadungeons have several appealing qualities. First, they are extremely easy to organize and facilitate. As many have pointed out, dungeons (mega or otherwise) are essentially narrative flow charts, with the decisions made in its exploration determining the course of the adventure. Unless they are deliberately designed as such, they are nearly impossible to "railroad," although most of WotC's free Game Day adventures with their point-to-point structures do exactly that. But what you don't have are gaming groups just running off the page to do whatever and you're making stuff up on the fly.
Second, good megadungeons can be organic. Monte Cook's DungeonaDay project has a nice quality, namely each encounter area has a "restock" listed, namely what is there when the PC's stop back a second time. Maybe it is just a replacement of the first encounter, but sometimes something very different has managed to inhabit the area. I like thinking about what the "restock" might be, because it moves the environment from being fairly static to something that is adaptive and living, which adds to its character and makes it more interesting.
Third, there's a certain classic, sentimental quality to it. Dark, dangerous, irredeemable evil--very rarely are there moral quandaries in a dungeon, and frankly I'm not a big fan of moral quandaries. Last Fall I played in a one-shot where we were gaming soldiers in the Middle East who were stranded when the US basically imploded into civil war. And I'll tell you, it was seven hours of nothing but bad choices. It's you against a world of monsters that could wipe you out in one fell swoop, so you're tackling them piecemeal and will take them down through a combination of luck and pluck. That hits my sweet spot.
Next: organizing the encounters.
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