(A brief edit to note: This is officially my 300th post on this blog. Wow.)
Zach Howard at the Zenopus Archives has managed to surprise us again, this time showing the original cross-section of Holmes's sample dungeon. Take a look and come back, I'll be talking about the hand-drawn image a good bit.
This little diagram is a master class in the "megadungeon in miniature." It condenses into 3 levels most of the principal ideas that underlie the Gygaxian megadungeon. There are multiple entrances, multiple connections between levels, different types of terrain, sloping passages, and generally everything you'd want from a complex dungeon, all in three neat levels. None of which is to disregard Tom Wham's great Skull Mountain diagram, but Holmes manages to do a lot with a few levels.
We come to two entrances on separate hills. This creates an interesting choice right off the bat. There might be something low-level like goblins or kobolds guarding the hillside entrance, but the descent into the mine is obviously the deeper way down. It's a good idea to let the rumor tables give a hint that the mine shaft to level 2 is a way down to more difficult monsters than the hillside cave.
In level 1 we're about equidistant from the two ways down. The level below the ladder has one of the long, gradual slopes that Gygax was fond of, and a party may reasonably be surprised when they go up a level of stairs and are still on the second level. Holmes's original wandering monster tables, drawn from Supplement I: Greyhawk, might give us a good idea of what kinds of threats lurk in each of them. Given how many humans with levels there are, it either suggests that the dungeon has a significant human faction, or is actively plied by rival adventuring parties. Either choice is interesting.
I really love those two carved-out areas by the mine shaft. They just have a ton of potential for mischief. As soon as I saw them I was envisioning a nasty monster swooping out as the PCs try to go down (or up) the ladder and causing all kinds of havoc. Or they could be rooms that are rigged with nasty traps, maybe something explosive, or a simple arrow trap that happens to knock the PC a long way down to the dungeon floor below. Or one of them could have a treasure visible, but a monster or a trap nearby that will turn the PCs' avarice into their undoing.
The third level's relative size suggests it should be a nice, big, sprawling level with lots of rooms and interesting tricks. Then there's the cave, and I have to admit if it had a lake indicated it'd be exactly after my heart. As it stands, the cave feels like it should really be the lair of a dragon as the culmination of the whole adventure (and a justification for the game being "Dungeons & Dragons"). It would really make the whole thing a summation of D&D in three levels.
It doesn't have the domed city and the great stone skull of Wham's drawing, but Holmes's original sketch points to a dungeon that can be played in the three character levels suggested by his rule book, and still give you the megadungeon experience. There's no reason the weird and cool stuff we've talked about previously can't be on these three levels, and Wham's drawing really only gives us an extra two or three true "levels" (counting 2 and 2A, the entrance cave as "one up", and the cave separate from the third normal dungeon level). What I think using a dungeon like this does is gives just enough of the complex elements without going into "megadungeon fatigue" that modern games often run into. By the time the players are bored with runs into the same dungeon, they're finished.
From that perspective, this is the missing link between the megadungeon and the smaller types that came to dominate the scene after the mid-1970s. You could describe this in a relatively short module but have months of play material come out of it. The only published dungeon that really comes close to this is Caverns of Thracia, whose reputation should tell you how great of a dungeon I think this could be turned into.