Megadungeon Musings, part 1

The subject of megadungeons is a perennial one, and events of the past few days have got me thinking about them again.  I've designed dungeons; I've designed large dungeons; but I've never designed a full-on megadungeon.  It's an intriguing exercise.

The two basic rules I'll follow are: 1) a minimum of 12 levels, not including sub-levels, demi-levels, quasi-levels, or any other sort of "not-a-full-on-level" that you want to name, and 2) no overarching theme or villain.  The most powerful monsters will be near the "bottom", but they won't control the entire dungeon.  Different factions will control different areas, with different goals.

I will map by hand, but I will also experiment with other formats, primarily sketch-up, to generate simple 3D visuals.

Preliminary features & details include the following:

• a gigantic vertical crevice or shaft, which provides the central focus of the dungeon.  The shaft has been closed over at certain points, so one cannot simply rappel from the first level to the 12th, but the shaft does span a number of levels, and not all of the closures are well maintained.  The shaft is large enough to allow fliers room to maneuver, and has links to the outside at its very highest reach.
• A pride of manticores controls the first 3 levels of the shaft.  The third level has a solid stone floor covering the shaft, and it is not apparent from above that the shaft continues below.
• The first level intersects the shaft roughly halfway between the "floor" and the highest visible point of the shaft.  Above the first level are the "Aerie Levels", a number of sub-levels that are mostly inaccessible except from the shaft.  Some also feature outdoor connections (onto a cliff-face or the like).
• At least the upper levels of the dungeon are within a mountain or other elevated terrain feature, rather than wholly below ground.  This allows some of the Aerie levels to include watchtowers or the like.