Ideas needed for a multi-level dungeon

[Spritelady]

I'm coming here in the hope that the wonderful community of Exilian can help me prepare an upcoming dungeon crawl in my DnD campaign (first part of which will be running this Sunday, to give an idea of the initial timeframe for me to get my thoughts on this organised). I find myself woefully short of time and, apparently, ideas for this particular session!

To give some brief context, my party are embarking on a noble quest to free a powerful spellcaster who is trapped and whose assistance they need. She is held captive by a powerful spell that requires two halves of a 'key' to unlock. The first half of the key is held in an underground complex designed to prevent the key being obtained. To add some complexity (and interesting monsters!), there is also a rift inside this area that has been allowing creatures etc to slip into the world from the Shadowfell. The area has long since been abandoned by the people who initially trapped the spellcaster, but it is still protected by whatever magical (or other) safeguards they left there.

In short, I need to prepare what I am hoping will be a relatively in-depth dungeon crawl for this underground complex, to challenge my players on their way to retrieve the first half of the key. I am considering pilfering the map of one (or more) levels from Waterdeep: Dungeon of the Mad Mage, and populating it with suitable puzzles/monsters/traps to challenge my players, but if anyone has better sources for an interesting multi-level dungeon map (ideally that don't require me doing much artistically beyond dropping some assets on top of it), that would be gratefully received.

So, the request! Does anyone have any good ideas for puzzles, traps, encounters etc that make for an interesting dungeon crawl? Any and all suggestions welcome!

[Jubal]

So, some thoughts for bigger dungeons:

• Pathing - the dungeon probably shouldn't be a linear system until maybe the "final run". Hidden pathways can be nice for the players to feel like they're exploring not just following a route. And that can work in 3D too if you have multiple levels.
• In general I think you don't want the dungeon to all feel the same, even if as in this case it's all a singularly conceived idea for a specific purpose. The two most obvious ways to add variation are, firstly, to make the original design vary: were multiple people planning this? Do they have their own sections or fingerprints, their own preferred monsters or ways of approaching the intruder problem? Secondly, there's the rift and general wear, tear, and breakage, so other things can have moved in.
• Traps aren't necessarily traps: that is to say, they're not necessarily intentionally placed things there to trap people. A broken section smashed through by a bullette or ruptured by an earthquake can have pitfalls, dangerous falling rocks, etc without anyone trapping it. Also there's of course lots of "trap monsters", though I think some of those are maybe better used almost in a themed section.
• Traps also aren't necessarily traps in the sense of doing physical damage. Teleport gates, charms, illusions, and so on are all pretty important elements of magical protection. Why bother killing adventurers if you can just direct them back out, or make them think they've won already? Simple bait-and-switches can also work: rather than the key half actually being in the obvious giant golden box or whatever, having it bricked into a wall or side pillar four rooms away and making the big shiny box a very large mimic would be a far more sensible move.
• Having some sidequest-type material helps. What else might each player character want down there? Are there potential allies they may need to find?
• Think who else is using the dungeon and why, beyond its intended purpose. Maybe a mage or a particularly introverted elemental or somesuch has found their way into part of it and is living there for the quiet. Perhaps there are some sections with very small tunnels where smaller humanoids might hang out.

Those are some initial thinkythinks, I'll try and come up with some more concrete ideas in the next couple of days.