A couple more things I learned about making a megadungeon.
Put most rooms off the hallways, not in the hallways. One thing I did a little too much of was have hallways end in rooms, and connect rooms in chains.
This isn't bad, per se, but it's limiting. It means that whatever needs to pass through that room - be it adventurers, wandering monsters or set monsters - must encounter what's in the room. When you random stock a trap in a room that blocks the only way into a sub-section, you're pretty much saying what lives in there has set the trap or is stuck because of it. If you have a monster or trap blocking the main thoroughfare, it affects what kind of monster or trap it can be.
The good side to this is it forces you to really think - maybe they pay a toll, or set that trap, or fly over it, etc. But I'd almost rather not have to. It also makes it hard to justify wandering monsters in some cases - behind the party is the entrance, and ahead are rooms with locked doors and traps and monsters, so where do those 1d6 skeletons wander from? Again, it forces you to plan more.
Plus having rooms set off to the side gives the party options - go in, or pass it by and risk leaving it in their 6. If it's the end of the hallway and it's forward or no-ward, they don't get to make that decision.
Have a lot of entrances, exits, and passages between levels. I have a number of ways into and out of the dungeon; two have been found so far and only one penetrated. There are others, and a fair amount of ways up and down. That said, the more the merrier. Put in a bunch, even if you close some off for a while with rubble, traps, platoons of elven guards, etc. It's easier to block an entrance than to add one later, and the more ways up and down the more options the players have. Plus, it means it's more and more plausible for monsters to get down there or get back out, for critters to find food, and for other adventuring parties to find their way down even if the players block the front way.
All that said, I'm still happy with what I've got, but I did have to change some encounters and modify the map a little in some areas to make sure these things happened.
The rest of the series are found here: