Ideas for a hard to get in library

[Doomwhispo]

Hello. Hope this is not misplaced here if so my apologies. I was thinking of making a mage mansion/library. I would like this place to be really hard to get in but not impossible. Something that would take my players a long time to figure out.
This place is run by a really powerful mage that's always in a bad mood. so magic is welcome. I don't really want traps that would do damage. Just stuff to keep intruders out.
Any ideas are welcome even bad ones might give me some inspiration.

[NecroDancer]

Perhaps an automatic suggestion spell that's tells people to leave. Or maybe a spell that mazes someone (D&D 5th edition maze not lady of pain maze).

[Biggstick]

What kind of things are you looking for the players to have to figure out to get in here?

When you said library, the first thing that came to my mind was Candlekeep. To get in to this fortress of a library, one has to present a book or piece of literature that is of a certain value that the keep doesn't have a copy of. Since Candlekeep has a huge repertoire of knowledge, finding a book like this could in of itself be considered quite an achievement.

[SillyPopeNachos]

Is the mage an illusionist? This would make it much easier. Say, for example, there's an area with a bunch of big pillars (10'x10' to 30'x30' base), if they're high enough level he will have Illusory Reality (to make the illusion of walls feel real when touched), and with Malleable Illusion the walls can shift, making the party think it's a shifting maze. They can get through it by A) dispel magic, or B) touching the walls at a later time.

[arrowed]

Glyph of Warding is your friend. Stone shape could seal doors when the PCs try to open them, Arcane Gates could drop them on to the ground outside the library, random books could polymorph them into chickens, which are then removed from the premises by preprogramed golems. Also, part of the challenge could be actually locating the library, with Mirage Arcane (if the mage is high enough level). Since the illusion is tactile, adventurers without truesight could get lost in one of the decoy libraries' mazes, only to eventually realise the true library is hidden from sight under a nearby hill (or illusion of a hill). Plus, you can actually use Guards and Wards! The PCs have 10 minutes after triggering an alarm to succeed in their infiltration, or every door, corridor and set of stairs becomes an obstacle. You can even have the structure under the effects permanently if you want!

[CursedRhubarb]

Some harmless things that could be fun:

A series of 5 square "entry rooms" where each has a door on each wall, each with a handle shaped like a different animal. All rooms look identical except for the floor color and are enchanted to clean the rooms of all markings and alterations once vacated so the party must proceed one room after another but doors must be passed through in a specific order. Wrong doors put them back in room 1. Once one door is opened in a room the other 3 will open to the same room until the doors are closed and the room vacated with the exception that the South door in room 1 always leads back outside.

A seemingly boring and empty hallway. (Short for fun, long to be evil) But upon touching the floor, walls, or ceiling whatever touches it becomes stuck as if glued. DC 15 strength check let's them pull the stuck part off but this means every step gets stuck, if they touch a wall it gets stuck, and poor souls that fall will need help to get up or a DC 17 check to sit up or push themselves up.

Memorization can get them through the rooms and determination or some spells/items will help with the hallway.

[Hopeless]

Depends on the library, for example they may need some item bearing a sigil that's recognised by the doorway allowing them access to the true library whilst any other method would only result in them finding themselves inside the actual building that's entirely separate from the library which is hidden inside either a pocket dimension or located beneath the building they're trying to enter...

Another possibility is that its built up to be some impenetrable place when the actual reason it can't be entered is because they need to activate some levers hidden nearby that would otherwise be unremarkable unless you knew what to look for.

So unless they bundle in the same time as a visitor they have no chance of penetrating the facility without some inside source especially one who can open it from the inside letting them in?

[eastmabl]

Puzzles might be the way to go. One cannot just murder their way into a puzzle.

[uraniumrooster]

You could have the Library exist in a Demiplane, with Glyphs that trigger Banishment on anyone who tries to enter without speaking the password (or whatever method of verification you find appropriate). It wouldn't hurt anybody, just send them back to their home plane.

[Specter]

For a second there I swear I read 'getting hard in a library'. Oh well.

Mordenkainen's Faithful Hound is an underrated spell which may help.

[Temperjoke]

Is it a "public" place, so to speak? I mean, is it intended that it's available for anyone to come to visit, but at the same time, only the worthy would be able to access? If so, you might consider a "hidden in plain sight" sort of entrance. The only way to get into the library is through a ramshackle, dilapidated restaurant, which has a door to the pantry. The pantry door has an illusion on it hiding a second door handle, which if that handle is used, causes the door to open into the library's lobby/antechamber, which is hidden in a special permanent demiplane. The restaurant is filled with illusions of customers and workers, mixed with a few guardian creatures who are covered with illusions.

If it's a private library, then something like out of Harry Potter would work, like how 12 Grimmauld Place was hidden from people not in the Order of the Phoenix; you couldn't see it at all to even attempt to enter if you weren't privy to it.

For either option, if the players want access to it, they can have a mini-quest to locate someone who could give them access (such as a key or charm that bypasses the defenses).

[BW022]

That completely depends upon the level of the wizard who runs the tower, why he wants to keep people out, and exactly how hard/long you want it to be for players to get in. Ideas in the more extreme...

There is no way in. You need some etherealness or astral travelling spell to get through the walls. Players need to go get some high level scrolls.

There is a door activated by a panel containing various small crystals. Players need to activate them in the order of various star constellations -- only visible in the opposite hemisphere.

There is a platform which teleports only small gold objects into a room. When the weight of gold in the room reaches 100 lbs (about 5,000gp) the door opens.

There is a door which never opens. The wizard doesn't live there, he lives in some secret cave nearby.

There is an imp inside which asks questions. The answers require the PCs to travel to various places, kill things, ask questions, etc. "How many teeth does a winter wolf have?", "If you are on the top of Mount Zimba at noon in mid-summer... what direction is the field of blue flowers and how far away is it?"

How nasty and/or long to you want the players to take solving it? Hours? Days? Months?

[Doomwhispo]

How nasty and/or long to you want the players to take solving it? Hours? Days? Months?
I'm looking for something they would not be able to solve/pass that same session. Something that they can keep on thinking about.
Thanks for the ideas so far keep them coming.