Honestly, if I were a high-level caster in DnD setting, I would only use Alarm. What could possibly be a better defense than myself? And if I’m not alive to recast the alarm, what do I care for the fate of my lab?

Would probably take inspiration from Marvel with The Collector (I think?) And show them buck wild shit with no defenses at all. If they want to rob someone who puts no locks on a lab full of expirements on undead dragons and crazy artifacts just sitting on tge wall, go for it.

That is the kinda DM I am though lol

What I did in an old C&S campaign was purloined lettered the Hell out of the lab.

The PCs invaded the wizard’s keep. They quickly searched the grounds, kicking down doors and making sure all the guards were subdued before hitting the main tower. There they fought their way up the tower to where the most defended room in the keep was. After disabling most (not all!) of the traps, magickal and physical both on the entrance way, and taking their lumps for the traps they missed, they broke into the proper lab, all filled with glassware and books and components and braziers and whatnot.

Only to have it turn out that everything in there was common and useless stuff glamoured up to seem important until you touched it.

In the mean time, while the PCs were being kept busy in the keep’s tower, the wizard came out of the basement crudely hidden under straw in the stables with all his most valuable stuff packed and carried out of the grounds to a safe distance before he triggered the final trap that turned the tower into a towering inferno.

The fools hadn’t even rolled to check for anything in the stables; I mean it was just the place where the horses were kept, right…?

If this a high fantasy setting where magic practice is common, then… I would pitch that like a modern day researcher might have very specialised domain knowledge they would most likely lack a deep understanding of security best practices but being aware of this would contact some external org to monitor and implement security for their lab.

They would likely need to present multiple factors to access it (something they have, are and know) eg when the lab is not in use it is folded into a pendant that the wizard carries with her at all times (something they have). To open the lab the dials on the pendant must be set to the correct positions (something they know) and the wizard must blow her breath (something they are) through the revealed channel, the only the person blowing and up to three willing people of her choice that she can see are then transported to the lab.

Tampering or unauthorised access countermeasures should be such that they minimise further exposure. Eg killing/hurting someone who has incorrectly set the dials is not the best strategy as the pendant is still out of the possession of the wizard. Instead maybe setting the dials incorrectly teleports the pendant back into the wizards pocket or, if teleportation is blocked, renders it inert until another physical factor is presented eg it is reset using the original workbench (at the security contractor’s offices) where it was made. Similarly if the wrong person’s breath is blown through the pendant then the person blowing is not transported into lab but to a random place on the same plane.

The wizard should be alerted to any possible unauthorised access so if the pendant is removed from their person or if the wrong dials or breath are used. Further access could require further authentication if it occurs in an unusual place or at an unusual time.

The vulnerability of the wizards security would then not be technical but human. Instead of trying to find the gaps in a diy security implementation the wizard has probably got frustrated with the multiple layers of security and self sabotaged things for convenience. Eg locked all but one dial in its correct setting so only one needs to be set each time; added an enchantment so that when the pendant is blown an illusion of the same time and place is projected to prevent further challenges; maybe even disabling alerts and counter measures since she has a pet that keeps stealing/fiddling with her pendant. The company that provides the security package might be aware of the hacks the wizard is using and could be in an ongoing argument with her, threatening to withdraw their services unless she stops bypassing security measures.

With this mystical artifact

Having it in a pocket realm in the Astral accessible by a mundane item off to the side in the ‘fake’ vault filled with low level junk magic items

The pocket realm also does not have any air becuase the BBEG is prepared with their own contingencies, and of course right on the other side of that airless door are a few very persistent golems there to hug anyone who enters

The access to the wizard’s vault is an escape room themed after the wizard’s life that the players learn of through advertising flyers.

The players will have to decipher clues placed in and outside the room, or interview NPCs, to determine useful facts about the wizard to solve the puzzles (names / # of siblings, favourite pet, first love, best friend’s sister’s name, etc.) - vary complexity / difficulty of research as needed. The wizard doesn’t care if they get in the vault; the wizard wants fame, entertainment, or are appeasing a deal with some entity.

Some puzzles may have lethal/deadly penalties - but really fun ones, like having to reach into a hole for a key, object, or to pick a lock, and it exchanges that limb for that of a fire elemental with no inherent protection from fire given to the rest of the body (temporary or permanent as level dictates); a failed puzzle guess results in a suggestion that elf blood will definitely open this lock (it doesn’t, but that must mean you need more, right?); or, just a bunch of dead bats that get dumped on them that later get animated from the dead or spider swarms explode from the corpses after 3 rounds (ghast bat swarm?).

Rooms that fill with water/lava/acid/rum/spikes are always fun. A book just filled with exploding runes in a variety of languages or that is in code and needs to be decoded first before they explode, each explosion larger than the last (book is always unharmed). Intermittent anti-magic zones (spells on even rounds only, temporarily for certain puzzles, spells create random effect of same level from an opposition school, etc.). They repeatedly find clues that point towards a need to kiss (escalate vigor and technique each time, require a certain number of spectators be rounded up, etc.) a statue in the town square, which has no apparent effect.

He re-stocks the vault often by hiring successful invaders for fun adventures that never require morally suspect behaviour of any sort.

Three doors, each leads to 20 doors. The correct door leads to 20 more doors in the correct area, the rest will lead to 20 more doors each, all traps. Some rooms are insulting, with no trap and a single copper on the floor. One of those rooms will lead to 20 more doors, one of which is the correct door.

Lead lined chamber underground, accessible only through a small teleport-only entryway guarded by golems.

• Weirdly the lab isn’t secured but it’s a real mess. However, the wizard being paranoid, they bring their job at home,.

• The good old paperwork nightmare sorry you need a pass A38 to enter

• A thrash but fun one, all the notes are tattooed on the back of “slaves”, if the players broke in by force, the slave would immediately escape, scattering the note in the city

Every see the mansions in scooby-doo? I always figured the whole reason D&D dungeons exists is some old wizard needed to project his workshop.

One of those Bugs Bunny / Yosemite Sam door-after-door-after-door affairs.

Basically have two short stretches of corridor with a door either end. The door at the far end of corridor 1 leads to corridor 2. The door at the far end of corridor 2 leads back to corridor 1.

The mechanisms triggering this are pressure plates along the centre of the corridor 2, skirting along the sides will avoid triggering the plates and allow you to go to the next room.

If a player makes a hole in the door / destroys the door entirely (which I’m pretty sure my players would), they’ll see the distant room magically change to the corridor when they step on the pressure plate.

Artifacts? In a museum, in the"lost and found" box. I tend to go with the wizard that is a bit careless over the super planned one.
The box may or may not be a mimic.

Research room: Wizard Itor uses the back room of his office. The front room contains mops and cleaning supplies (some very magical). The door is even labeled “Jan Itor Supplies”

It looks like a dingy old library. The librarian is actually a genie using mind-control-illusion on everybody but the wizard.

It is a really busy lab where assistants move stuff and build things 24/7. Everybody as an alarm-switch-gadget.

The Eternal Vault is intended for long term storage: Only creatures can get out. No thing, no gear, no spells. The wizard strips naked whenever he needs to look something up in there.

The wizard has an elaborate tapestry on the wall. When under the light of a specific magical candle, you can enter the tapestry to a pocket dimension where everything is woven. There you can find his vault of goodies, literally removed from reality without the light of the candle. If they take the tapestry as a whole, they have to recreate the exact proportions of the wizards study to make it work again.

I’d put the lab in a pocket dimension, accessible only through something on the wizard’s person. Like a really big bag of holding, but with an air supply.

I’d also rule that pocket dimensions are accessible by an arduous trek through some kind of weirdspace.