Sean McCoy of Mothership fame has proposed a daily exercise called #Dungeon23. As he originally tweeted (in part) on 5 December 2022, "Megadungeon for 2023. 12 levels. 365 rooms. One room a day. Keep it all in a journal." One of his tips is, "Don’t overthink it. Don’t make a grand plan, just sit down each day and focus on writing a good dungeon room." That's my first stumbling block: Don't overthink it. I don't think I'm capable of not overthinking it. I need to know who made it, what function it serves (and perhaps what function it was originally intended to serve), where it is located, when it was built, how it was built, how adventurers will learn of its existence, and what is supposed to be there that will motivate adventurers to explore it. All of this information is essential to my design, but the advice is "Don't overthink it. Don't make a grand plan..." Without a grand plan, however, it will likely end up being a nonsensical funhouse dungeon, which is fine if that's your objective, but if that's the extent of your ambition, why bother to chart it in advance? Why not stock the dungeon as play progresses rather than wasting time on scores of rooms the player characters may never visit? You could devise the floorplan the same way, just barely ahead of their light source. You could even do it randomly via the 1st edition Advanced Dungeons & Dragons Dungeon Masters Guide (Appendix A: Random Dungeon Generation, pp. 169-173).
Of course someone like me will create a grand plan. That's where I find my motivation to create an environment for adventure. For exploration to be worthwhile, there must be reasons for things. There must be patterns that can be discerned by those who are observant. There must be facts that can be deduced by those with intelligence. Everything flows from purpose. Without a grand plan to guide my design, I might as well generate the entire dungeon with a click of the mouse.
This is not to discourage anyone who enjoys funhouse dungeons from creating their own fungeon one room at a time. This just speaks of my own inability to participate. Even if I can accept the funhouse aspect of it, I still need to rationalize why it exists the way it does, which leads me to the classic answer: mad archmages. Nearly anything that defies logic can be attributed to the whims of insane spellcasters. This might be a good excuse to revisit my old satirical setting, The World of Greyauk. Perhaps it is time to think about finally designing that megadungeon beneath Castle Greyauk...