In a saga I'm in my character slowly slides down the deep end of paranoia and preparedness. Yet, his sanctum only has a latched wooden door: up to not so long ago he plainly didn't kept any possesions.
However, this is about to change.
Now, how could he protect his possesions from an intrusion of a hermetic, who very likely would have effectively unlimited time to subject his sanctum to a thorought search.
At first, l though "ok, circular ward against humans, mighty creaturs, animals", but even if Pen would be sufficient (and charged items are great, so), hermetic can fairly easily dispel it or act across it. Aegis can be mastered to have doubled resistance, so it's a non-factor in preventing use of magic.
Then, l thought of an effect alike a Shrouded Glen (ReMe to not allow anyone find a place in a boundary) with target Room and some duration to hide most prized possessions. But hernetic can identify such an effect and dispel it.
So, how can you protect a sanctum from a hermetic enemy?
How your Magus protects their sanctum from other Magi works much like you protecting your home in the real world.
What I mean by that is the normal and reasonable stuff really only stops the honest and dissuades the capable. For the vast majority of Magi the primary defense of their sanctum is themselves. They generally do not want to spend, or possibly have, the resources (Vis and Time) required to make the sanctum more secure and even the ones who do spend some of those resources generally only slightly slow down a powerful Hermetic.
The thing is every season and pawn of Vis spent on improving the security of a Sanctum is a season and pawn of Vis that could be used instead to improve the power of a Magus. Early in their life the lack of power seriously restricts what they can accomplish and pretty much nothing they do will slow down a powerful Hermetic. Late in life when they have enough power to create things that can inconvenience Hermetics they are at the point where they have to spend several seasons to increase their power or work on their legacy.
And no matter what you do, highly skilled and powered opposition can always overcome the defenses if they have unlimited time to work on them.
So rather than protecting your Sanctum, how can we protect a collection of items? Again looking to the real world how do governments protect their most powerful and secret stuff? By hiding it in remote locations that as few as possible know about or have access to. So we need to apply that and create a 'hidden vault'.
First your Magus should do everything for it himself if possible, which cuts down on the people who know about it to one. It needs to be as remote and impossible to stumble onto as can be achieved. The best possible is somewhere than can only be accessed by AC teleport. A small room in the middle of a mountain or deep underground with at least a mile of stone in any direction that only you know about is unlikely to ever be found.
I haven't done this before, so take my suggestions with a grain of salt.
First thing: The inside of your sanctum is effectively a no go zone for other Magi, by Peripheral Code ruling. "Technically" you can do anything to intruders. Hermetic Oath is all but suspended.
If you expect magical dispelling, keep a mundane threath under a continuous magical miniturisation.
Mechanical traps don't need to penetrate Parma Magica. But magic should be Okay to automatically reset them.
Make your traps out of multiple Forms. eg the spiked pit includes stakes made of Wood, metal, animal bone, and human bone. And if you keep the pit artificially cold, stakes can be also made of ice.
Have an apothecary add poison to your traps - a gentle mist of aconite if a pressure plate has not been disabled by a magic word..
Claim your trap is part of your lab systems - you magically keep a tub of water below freezing in rhe ceiling for airconditioning, that is setup to drop a tonne of ice if a trip swtch is tripped.
Secretly keep a trained watch animal in your sanctum - unleash the wolverine.
Of course, your mage now needs to remember how all the traps are tripped, and avoid setting them off.
My (beloved and wonderful) maga once wrote a tractatus on this subject (Never finished, although I did a good deal and it was fun).
To sum it up: If the intruder has enough time and ressources, you will "lose" in the end. Even against mundanes.
So, as @InfinityzeN said, the best solution is just secrecy.
Before that, one thing she advises is to have 2 kind of effects:
Those that work unless dispelled
Those that work if dispelled.
(Well, actually, she uses 3 types of effects, but this is the most important distinction)
For a simple example, if the ceiling is held by magic, dispelling magic in the room is not a great idea.
Reminds me that modern day bank vaults are rated in terms of how much uninterrupted time it will take to break in.
Quite a few heist movies have plot points where the crims have to carry out elaborate plans to ensure they get enough uninterrupted time to penetrate the vault.
The whole point of a bank vault is that it is a time delay for the actual security to arrive, which is the people who show up with the pewpews. That is why I said for most Magi the primary defense of their sanctum is themselves. Magi are the big pewpews in Ars Magica. Something a Magi can't handle is too big for almost anything to handle.
Of course having a Flambeau in your Covenant who is willing to provide some "Fire"power against invaders is always a big help. Almost no one is stupid enough to invade the sanctum of a Flambeau combat monster who is actually in it. You just provide him with a momentary distraction and free pop-up target.
It is, as I said, incomplete, but it was fun to write a document "in character". I hope you somewhat enjoy it. There is it (Luckily, I saved it on the wiki)
