From the characters’ perspective, whence comes this natural law that dungeon level 2 is harder than dungeon level 1, and so on?
Here and there, bloggers toy with the idea of the dungeon as a mythic underworld, an actively hostile place with its own rules. This makes sense of the Gygaxian dungeon’s changing layout and the favoritism it shows towards monsters (they can see in the dark and don’t need to force doors). It doesn’t really explain why deeper levels are harder. If the dungeon wants to kill people, why not have level-10 death traps on level 1? If it wants to lure people deeper, why not just have a trail of coins leading to level 10?
How about this: the forward edge of a dungeon wriggles though the earth like a snake, leaving skins behind. The living stone of its chaotic creation is on the deepest levels, those that Gygax refers to in Underworld and Wilderness Adventures as “under construction.” Imagine corridors writhing through the earth, doors budding from walls. The shallower levels are the snake skins, each shed by a younger and weaker version of the dungeon, and each with a relatively fixed map. As you descend into the dungeon, you find archaeological evidence of its increasing wealth, cunning, and strength, in the form of more treasure, more dangerous traps, and stronger autochthonous monsters. (The dragon that’s too big to leave its dungeon room is born of and sustained by the living stone.)
Every once in a while, the snake revisits upper levels, leaving a changed floor plan and new challenges in its wake. For the most part, though, it delves ever downward. Perhaps its increasing power is fueled by the XP it earns killing adventurers.
I don’t envision the living dungeon as manifesting as a literal snake. Instead it’s a nightmarish ever-changing zone of self-digging tunnels, doors that turn into stone walls behind you, and monsters oozing from walls. It can be killed, perhaps, by sunlight, which is a hard commodity to ship to level 10 of a dungeon.