Last week I talked about megadungeons being focused exclusively on maximizing the time you actually spend playing at the table. Zak recently had a post titled All Mouths Lead to One Stomach about how all dungeons are connected.
The truth is, in an abstract philosophical sense, every place that you create for your players to explore is really part of the same dungeon - the dungeon of their experience of playing with you. All content is your content. You are the environment they explore.
Portals (such as those in the Incunabulum of the Uncanny Gates) were a convenient tool to provide a varied play experience. Some of the only real, detailed, professionally published information about what the original Castle Greyhawk was like come from modules like EX1: Dungeonland, which outline specific sub-levels in the ancient Castle.
This sub-level structure was a clear way to get all the advantages of Megadungeon play, while still allowing some more traditional advantages of an above ground hexcrawl, providing a variety of play experiences. This design aesthetic of the 'sub-level' is carried over into more traditional modules like the T1-4: The Temple of Elemental Evil and C2: Ghost Tower of Inverness, both of which use elemental sub-levels to break up the adventure.
What is a sub-level?They are typically smaller than traditional levels. They are usually remote and somewhat difficult to access. They rarely lead to new sections of the dungeon, sometimes being labeled with a number indicating their depth, and a letter indicating that they are a sub-level (e.g. Level 2b). They are generally more difficult challenges than the surrounding environment, and often have much better treasure as well, but the challenges are often unique (e.g. environments made of elements, anti-gravity, anti-magic, etc.)
The general conceit is that it departs from the progression of the dungeon. The divide between a sub-level and level is, of course, ultimately arbitrary.