Here are some more ramblings about dungeon design, specifically megadungeon design. This is just what I've discovered as I have gone along.
The rest of the series is linked from here.
Quick Felltower/Grak Yorl Status Update
My own megadungeon has, as of now, 4 main levels and 1 sub-level mapped out, and everything the players could possibly reach in any two sessions is stocked and ready. I've got notes and outsides on several more levels and sub levels, and I'll spend time on them when I've got a killer idea or the players seem like they'll get there within a session or two.
On to the lessons:
Know which way the doors open. When I started my megadungeon mapping, I went right back to Gygaxian mapping, with doors shown as boxes overlaying the wall.
I realize now I missed a good chance to make my life easier. I should have put architectural blueprint style notations of which way a door swings open.
If your players are anything like mine, the first thing they door when they see a closed door is ask, which side are the hinges on? No locked, barred, or magically sealed door or poison needle doorhandle trap is worth beans if my players are on the side with the hinges. Hinges come off. That's if they don't follow the tactics of Spec Ops troops and make their own entrance and ignore that silly choke point.
Which way the door swings - from left to right, or right to left, into the room or out - will affect every decision you make about the door. Do you bust it down? Would there be a bar on the inside, or could someone have piled trash up to keep it from being shouldered open? That would make sense if it swings into the room, but not as much if it opens out. If it has recessed hinges or is a pocket door or swings both ways, it changes the nature of the door (and the wall next to it).
While I can make this up on the fly from knowing what my dungeon doors were put up for, and why, it would have been easier from the start to have it noted on the map. That way I don't have to remember.
Make it easy to change levels. It shouldn't be a nightmare to get down to level 2 from level 1. It doesn't need to be trivial, but the smoother the transit, the more chances that players will actually go there. You want to avoid making it so that every attempt to get down to the next level involves a resource-draining slog or takes up a big chunk of your game session. It's just logistics - if going deeper is in and of itself costly and time-consuming, it won't happen as often.
Two corollaries to this:
Make it possible to bypass sections, but give them a reason not to. If it's easy to, say, skip from level 3 down to level 7 using the elevator, or the big access shaft, or the teleporter, great. But then you need a reason to go to levels 4, 5, and 6 instead of doing a quick raid on level 7. Besides the inherent danger of going too deep, that is. Put things - valuable, unique, and interesting things - on levels 4, 5, and 6 that make it worth exploring them. Put dangerous creatures there that must be dealt with to avoid leaving monsters in your six. Put cool stuff there.
I played the hell out of Wizardy: Proving Grounds of the Mad Overlord (available here) when I was a kid. There was an elevator from level 1 to level 4, and then from 4 or 5 (I forget which) down to 9. So I never spent time on levels 6, 7, or 8. They were filled with teleporters, turnabout traps, mazes, and nastiness. But I could just ride down to 9, fight one all-or-nothing encounter (and let rip with my best spells and magic items), take my loot, and run back to town. So I did that over and over - it was risky, but the level 9 critters were worth a lot of XP and we found the best magic items there. All 6-8 had was less treasure and more frustration. So avoid that!
Include subtle level changes
I'm not a big fan of teleporters, especially the kind that do it without you ever noticing (I find that hard to believe in, and it wrecks my suspension of disbelief). But I do like stairs down to sub levels, or sloping passages, or raised up or lowered areas on a level. Combined, they make it hard to tell what's a passage to the next level and what's just part of this level but raised up or lowered down. If you go down to level 7 and up a tall set of stairs to the temple's alter and priests chambers, is it still level 7 in danger and reward? Does that sloping passage in the caves on level 5 drop you down to more dangerous stuff, or does it merely mean a physically lower area of level 5?
Man-made dungeon levels might take advantage of a dead cave found during the excavation, and it might be above or below the rest of the level.
Putting that kind of conundrum in from of the players tests their willingness to explore. It adds tension and choices with small cost. It also adds realism - caves aren't flat, and having to climb up and down obstacles makes a lot of sense.