A few posts have value that goes beyond the immediate context in which they were posted. Al's (Beyond the Black Gate) Megadungeon Design and Philosophy is required reading for anyone designing or playing in a megadungeon. It is presented in three parts: Part 1, Part 2, and Part 3.
Rather than present rules, Al gives an overall philosophical framework for megadungeon design. While I have developed my own approach to designing a megadungeon (basically "Holy Crap, it's Monday night again!"), I go back to his three posts again and again.
Al also has conveniently posted a Megadungeons Resources, with links to his own posts and other online resources. Excellent stuff!