Generic high-fantasy setting (d&d 5e if you want to use specific spells), what security or defences would mages use to secure the place where they keep all their magical stuff? I’m thinking decoys, reinforced and hidden location, guards (hired or summoned), locks (both magical and mundane), booby traps, and spells to discourage or confuse passing snoopers.

Also it’s going to vary depending on the resources of the wizard and of the players. Is this an independent student wizard protecting their studies that a low-level party could reasonably break into, or is it the king’s secure vault of confiscated magical horrors that’s a final boss for the campaign?

I’m looking for ideas for fun obstacles, and critically that the players can’t just sidestep with a single low-level spell!