I think a boobytrapped dungeon is more secure than a demiplane and etc to hide objects of importance such as a spellbook to copy spells from if the working book is lost or ruined.

Thoughts about a dungeon being a more secure location to hide things?

I've always struggled with the idea of dungeons themselves because if there's enough value there then why not just pitmine the entire dungeon to destroy it traps/creatures and all. Like digging up an anthill.

But preventing that seems better than preventing outright theft from a demiplane.

Sometimes the best way to keep something safe isn’t necessarily to fill it with traps, it’s just to have it in a really well hidden location. If a wizard constructed a dungeon in the darkest and furthest reaches of the land where nobody ever goes, and then dies with the secret of its location, it could be hundreds of years until anyone stumbles upon it, and then they have the challenge of beating it. Even better, you could have this dungeon be known but have its location secret, so the adventurers will encounter lots of enemies and hazards on their way to find and discover the dungeon, effectively turning the whole adventure and map into a dungeon-like situation without even knowing where the real dungeon is. Suddenly what was once a session-long dungeon crawl to get something is now the focus of a whole campaign just because you’ve removed that one key detail from the NPCs collective knowledge, effectively making the dungeon far more difficult than it was before 

Demiplane + Sequester

Realistically, no, dungeons are not a good choice for protecting anything. In genre, however, wizards would absolutely use dungeons.

You're living in a world with no television, movies or internet. And you're an introvert. So you've got a lot of time on your hands. What to do?

Build your very own dungeon. Spread the word. Watch as hapless adventurers flail against the traps and puzzles you've made. Hours, if not days, worth of entertainment. The best part is, when they fail they drop gold, rations and equipment. It's a lucrative income opportunity! Sure, eventually some group is going to clear the dungeon. But they're just getting the loot that was brought to you by their predecessors. 

Quote from Rathkryn >>

You're living in a world with no television, movies or internet. And you're an introvert. So you've got a lot of time on your hands. What to do?

Build your very own dungeon. Spread the word. Watch as hapless adventurers flail against the traps and puzzles you've made. Hours, if not days, worth of entertainment. The best part is, when they fail they drop gold, rations and equipment. It's a lucrative income opportunity! Sure, eventually some group is going to clear the dungeon. But they're just getting the loot that was brought to you by their predecessors. 

Way back in the AD&D days, as a bridge between the Against the Giants and the Drow modules, I came up an extremely powerful wizard who built three different maze-like dungeons in the Underdark as experiments, just to observe whoever stumbled into them

Wizards have been hiding stuff in dungeons since 1974.

I don't know.  If I were living in a world full of professional dungeon delvers who weekly plumb the depths of dungeons, I'd think twice about using a dungeon to hide something valuable.

As per the evil overlord list, I'd more likely hide a false valuable in a dungeon and keep the real one in the local bank. 

Banks don't seem to be robbed nearly as often as tombs and dungeons in the worlds of Dungeons and Dragons.

It seems to me that large works of engineering will be more expensive than the items they're meant to protect. A castle protects a region around it, and thus is an investment in taxes and other income for generations (as far as the local count knows, for all eternity).

Wizards constructing dungeons to protect stuff seems ... sketchy. An entrepreneurial wizard might construct extensive workshops and laboratories, but it should be noted that unless he has some form of income, they would be really small. So such a wizard (being entrepreneurial) would run a business: Creating monsters or golems, breeding mutants and halfbloods, for someone. Anyone. Specifically, propably someone of questionable morals - or the king, which is almost assuredly the same. 

So let's say the Tomb of Horrors isn't reasonable. If we accept it's existance, we must assume that the wizard who made was a madman. Which he was, I guess. But he didn't construct the place to protect his stuff, he built it either out of compulsive need, or out of a desire to torment adventurers. 

Sane wizards would protect their stuff by making copies, or putting items in a vault at the local Dwarf Bank franchise. 

I think with magic a wizard would be able to construct a dungeon much cheaper and easier than people would imagine. Mold Earth is a cantrip that can excavate and move soil, rubble and debris, so a wizard could easily create an artificial cave or basic underground structure within a day or two. Populating it with monsters usually happens naturally but a wizard could cast Hold Monster to relocate them to that position, and there are all sorts of simple traps that can be created with magic. I’m probably missing a lot but there are so many spells that would make the construction of a dungeon really easy and it would barely cost anything.

I think ‘dungeon’ has became a very, very loose term for any structure that an adventurer would raid. A dungeon could be literally anything as long as it’s got enough peril and danger for it to be classed as a threat for adventurers to overcome. You could take a regular home, throw some spikes over the floor, put a monster in the kitchen, hide something valuable within and now it counts as a dungeon by dnd standards. I think that highlights how a wizard usually wouldn’t make a dungeon from scratch, they’d take an old tower or crumbling ruins and just convert it, rather than making a whole compound themselves 

The best place for a Wizard or any magic-user to put their most prized valuables?  Inside your own body. Replace one of your less vital organs with an container that does not get degraded by contact with blood or muscle. Shrink the container and put it into the place where your organ used to be. Then cast Modify Memory on the NPC surgeon who helped you put it there. Make sure you pay that person well, but not too well. The most important part is: never talk about it and never think about it when you are in a room with anyone who might be a spellcaster.