>They've got seven different instances of "if you cast this spell every day in the same spot for a year, it becomes permanent" on their schedule, and they've been doing that for centuries.
Every corridor of their lair is filled with heavily-obscuring fog. Also, that fog is Stinking Cloud, which means you are Con saving literally every round to not barf. This Stinking Cloud is E V E R Y W H E R E and comes back within minutes if dissipated. But you can't dissipate it, because every corridor also has a fucking Gust of Wind blowing straight at you, at all times, and it's blowing in Stinking Cloud from everywhere else. Every bit of air in this place is fucking Stinking Cloud.
Any time you make a turn in these Stinking Cloud-filled halls, there's a 50% chance you go in a direction you didn't want to, into another hallway filled with Stinking Cloud and Gust of Wind pushing you back.
Every door is Arcane Locked, and is illusioned to look like the wall. Also, covered up by Stinking Cloud.
Staircases are filled with regenerating Web. Also, Stinking Cloud.
There are Magic Mouths every five feet, screaming "GET THE FUCK OUT". You can only hear the nearest Magic Mouth, because every tile is partitioned off by a sound-proof barrier. This partition is also vision-proof, meaning that even if everything wasn't a swirling mass of Stinking Cloud, you'd still be blind every five feet.
Any time you turn a corner or enter a room by the door you couldn't see to begin with, you must Wisdom save against Suggestion telling you, "It'd be a good idea to leave and go home." EVERY TIME.
Divination magic doesn't work. Divination sensors cast outside can't come in, either.
Teleportation and planar travel magic doesn't work.
It's nothing but 5x5 cubes of sound-proof, vision-proof, difficult terrain vomit gas screaming GET THE FUCK OUT.
And that's before we even talk about the traps rigged to pressure plates and tripwires that you can't possibly see.