One of the things I here while the players are going over the map of Felltower is "that's where the ____ lives" or "That's where the ____ is."
That's not always true, though, and it can mean the players are put off of their plans when they arrive and it or they are not there.
The architecture is really the most fixed element in Felltower. Even that's a bit changeable.
With a megadungeon I feel like you need a good base of consistency, but that consistency can't mean that what's in one room when you arrive the first time will forever and always be in that room. While the orcs may always be in one area, the undead may congregate in another, and that one area always seems to be plagued by rats and slimes . . . that doesn't mean you won't find those things in other places.
And it doesn't account for wandering monsters or reactive monsters.
A good example of that was the werewolves. They were dwelling in one area until the PCs showed up and cut down a lot of them. They went back to finish the job and they were gone. Well, yes. They'd taken some very heavy casualties and moved to a lower-risk area (at least as they perceived it to be.) They didn't move far enough, but it may be that something was keeping them in that area instead of another.
Sometimes you can make the mistake of thinking that where you ran into something was where something lives. That kind of error is why "the Lord of Spite's apartment" actually turned out to be stairs down to a whole level where the Lord of Spite actually seems to "live." Turns out he was out for a stroll, not living behind that one door.
Plus the state of monsters changes. The dragon isn't always sleeping, the orcs are always on alert by the main entrance, and so on.
The terrain is largely consistent, much more so than the monsters, but even so . . . sometimes hallways are blocked. Sometimes they're open. Doors are open or they are closed. Bars get installed. New tunnels get dug, by animal-intelligence monsters and sapient dwellers alike.
You can depend on many things in Felltower, but over time things change. You can't always expect to meet or find the things you met or found where you met or found them. Usually you do, but you can't bet your trip on any one thing being as it was. Or especially on all of the things beings as they were.