What is the Great Dungeon?

The Great Dungeon is an unabashedly retro-style megadungeon setting I've been running off-and-on since 2010. I wanted something fun and casual that I could pick up, put down, drop random groups into when needed. It was initially built for D&D 3.5e but pieces were adapted for Swords and Wizardry and the Pathfinder Beginner's Box. In the last couple of years I've been running it for 5e and I'm gradually porting older pieces over to the new system.

I have done past campaigns that tried to emulate one fantasy subgenre or another, but my primary goal for the Great Dungeon was to emulate nothing more or less than classic D&D itself: wild, shaggy, gamist. My primary influences for the setting, in order:

1. OD&D with its emphasis on megadungeon and wilderness adventuring. I wanted the setting to contain anything and everything that could be found in the three LBB, with less reliance on subsequent supplements.
2. Holmes, as a cleaned up and polished expression of OD&D
3. Moldvay/Cook, for being the version nearest and dearest to my heart;
4. AD&D, though I wanted to make it a much smaller influence, with less emphasis on the Great Wheel cosmology and the nine alignments. Focus on the first three core books plus Fiend Folio (which was really an OD&D supplement at heart). No Unearthed Arcana, no drow.
5. Select elements from later editions, if I liked them and felt they fit with that weird OD&D vibe. These are admittedly idiosyncratic decisions. So gnomes and tieflings as PCs, yes. Dragonborn, warforged, no.

Although I never played OD&D, one of the first supplements I bought after the Basic and Expert Moldvay/Cook sets was the Best of the Dragon, Vol I, a compilation of articles from the Strategic Review and the first couple years of the Dragon magazine. The material was all written for OD&D and helped convey a lot of the early game's aesthetic.

In the last six or seven years several bloggers have done a nice job poking at the implied setting behind (below?) D&D, and these inquiries have greatly helped shape my thinking about the Great Dungeon. One of the best series was written by Wayne Rossi at Semper Initiativus Unum, who noted "If you actually read the wilderness description in OD&D volume 3: The Underworld & Wilderness Adventures, it turns out that the implied details of the setting are weird." As a result, I wanted the setting to have a few distinct features:

1. Post-apocalyptic. Jeff Rients might have been one of the first to assert that "Default D&D is post-apocalyptic in nature," but several others such as Hill Cantons drilled down with posts like "What Rough Men Tell Us About AD&D's Implied World."  So the setting of the Great Dungeon is built on a series of older civilizations, with Norumbega being the most significant, but with two more recent calamities resulting in bandits, brigands, and other unsavory fellows lurking in the wild lands. Which brings me to the next point . . .
2. Wilderness. The Outdoor Survival map was really an unofficial OD&D supplement, and hexcrawling through vast, dangerous wilderlands is an important aspect of the game. Delta recently demonstrated how influential Outdoor Survival was on OD&D rules for exploring and movement. In OD&D the wilderness is important not just as an opportunity for adventure but also as a resource for high level adventurers to exploit in order to establish their own freeholds. So it was important to place the Great Dungeon in the North,  a wild and largely unclaimed region.
3. Only Semi-feudal and Quasi-medieval. There are swords and horses,  sure. But the resemblance to medieval Europe is purely superficial, as Blog of Holding recently detailed in "D&D is Anti-Medieval." Class and religion in the North are not the monolithic forces you would find in medieval Europe. Social mobility and travel is common and encouraged. In practice the D&D setting is much closer to the American West of the 19th century than, say, France of the 12th.