Creating a Fortress that Allows Owner to Scry, but Blocks Others

Question

I'm a GM designing the remote fortress of an ancient-but-still-living archmage (>=20th level?). The archmage lives in society at present, and only returns to his fortress a few times per year, or as needed, via a permanent teleportation circle.

Previously, the archmage hid in this fortress for centuries to escape persecution from...well...pretty much everyone, so the fortress needs to be protected against even high-level divination magic. For this reason, I plan to line the fortress with a layer of lead and have some permanent spells like Mage's Private Sanctum, Nondetection, and similar, to prevent most/all divination magic.

At the same time, because the archmage has begun living in society again (in disguise), it makes sense that he would place some kind of alarm that would warn him, even at great distances, of intruders. Furthermore, it makes sense that he'd have a sort of permanent scrying system set up so that he alone could observe what's going on in his fortress.

(Note: The archmage's INT score is way above my own, so I figured I'd do the best I can by crowdsourcing...)

My Question

• Is there any rules-as-written avenue for the archmage to simultaneously prevent others from successfully using divination spells on his fortress but also permit divination spells of his own?
  • If so, how could that be done?
  • If not, what might be the closest thing to it?

Answer 1

The Mage's Private Sanctum spell you cited does actually the job, and is probably your best solution: it makes detection impossible, thus there is no need for any alarm.

This spell ensures privacy. Anyone looking into the area from outside sees only a dark, foggy mass. Darkvision cannot penetrate it. No sounds, no matter how loud, can escape the area, so nobody can eavesdrop from outside. Those inside can see out normally.

Divination (scrying) spells cannot perceive anything within the area, and those within are immune to detect thoughts. The ward prevents speech between those inside and those outside (because it blocks sound), but it does not prevent other communication, such as a sending or message spell, or telepathic communication, such as that between a wizard and his familiar.(...)

Mage's private sanctum can be made permanent with a permanency spell.

So basically: Makes everyone inside immune to detection of any kind from outside. It can be made permanent (which is not the case of Nondetection), doesn't rely on saves, and shouldn't block your own scryings, unless you are targeting something which is also inside the sanctum. ("Divination (scrying) spells cannot perceive anything within the area" could be read as "casting a scrying spell from inside the area won't work" but I am convinced that it is not the intended reading). The limited area means the archmage may have to cast it several times, depending on its manor's size.

All the other ways to escape scrying either just make scrying more difficult (e.g. Hide-Out), or have a limited duration (e.g. Nondetection), or make the archmage have to go out of his lair to scry (e.g. Create Demiplane with the dead magic feature)

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As a sidenote there is nothing such as people above the 20th level in RAW Pathfinder. The 20th is supposed to be a hard cap.

However there is a system of mythic tiers that can be used for this kind of things. If you want to insist on how powerful this archmage is you can give him tiers in the Archmage mythic path.

An Archmage with a tier of at least 3 can access Reverse Scrying, which doesn't help for what you are looking for but makes him able to return the scry to who was spying on him in the first place. Such a paranoid mage would probably take this path ability.

Answer 2

Have him cast a lot of greater alarm and let him make them permanent with the permanency spell. let him cast telepathy on minions every day or give them a helm of telepathy and give them the instruction to inform him if they hear the alarm or detect intruders through other means. Now let him hide the greater alarms with arcanist magic aura. Let him install mechanical traps that make noise if they go off and are hidden by arcanist magical aura to detect creatures that use arcanist magical aura to avoid his alarm spells.

problems that arise are:
as long as he wants to be able to teleport back to deal with the intruder a teleporter can surpass alarms, so unless he gives up his ability to teleport everywehre in his lair he or has permanent surveillance by true seeing creatures, he has places in the lair he can't detect Lucky teleporters.
Another problem are parties and multiclassers. Unless he has constant surveillance by true seeing creatures a party with a high level rogue and a mediocre level wizard can enter his lair undetected.

For now I only have the fix that he can use Otiluke's suppressing field to surpress the use of arcanist magical aura, dispelling magic, wish, limited wish and surpression magic, but this spell is very temporary, so he either has some very active minions or other spell restricting magic like anti-magic field.

I've found more high level and more efficient ways to deal with this.
Give him a focus, a lot of (possibly death) high level friends, similacrums or 10th level spells before Mystra's Ban in the forgotten realms
Let him cast living vault
Let him prepare a simulacrum spell, sending and a lot of detection spells or
Let him cast his simulacrum
Let him command his simulacrum to warn him if there is an intruder
Let him set one permanent greater alarms with an audio signal
Let him use teleport restricting magic or purchase a weirdstone and aply it to his lair
Let him protect the alarms with (possibly permanent) prismatic spheres
Let his simulacrum cast anti-magic field near the prismatic sphere and wait.

Answer 3

There's a wizard discovery that's perfect for this: https://www.d20pfsrd.com/classes/core-classes/wizard/arcane-discoveries/arcane-discoveries-paizo/observant-illusion-su/

Observant Illusion, a Wizard discovery available at 9th level. It allows you to see from the perspective of a figment or shadow illusion that you created, whenever you like, and is not a divination effect. So you can ward your fortress up with anti-divination no problem.

What's a handy shadow effect that lasts forever? Shadow Evocation: Continual Flame. Make all the lighting in your fortress use these, and you can look into any room easily. It's not even suspicious that you'd use an illusion for this, because it avoids the costly material component.

Also a shadow effect - Simulacrum. As in, the things a high-level isolated Wizard probably already has as servants and guards at their fortress.

And of course, permanent images - how about some illusory windows to make the rooms more cheerful? Now they double as spy cameras.

Also, while it won't let you directly see, you can get the report of a trusted minion about goings on at your fortress just by having a permanent Telepathic Bond set up with them.