Always.
Vocis ponders for a bit, then says Well, that's certainly a challenge. To address your proposals first, yes, illusions to mask buildings or the covenant itself could certainly be developed. My concern is that a determined enemy could bypass these rather quickly. This is doubly true if the enemy is already familiar with the layout or location of the covenant. If our goal is to defend against the same assailants that attacked in 1229, we'll need something much more complex.
I'm going to run through my ideas OOC for simplicity. I'm also going to ignore the details of any linked triggers. We can sort that out later.
Masking any of the buildings is a pretty simple MuIm effect. There's a spell in the NWG that conceals a castle at level 20. Even accounting for any NWG-related not-quite-rightness, that's something Vocis can manage pretty easily. Masking the covenant on short notice would be difficult/impossible, since it will (presumably) require a Boundary target and thus a ritual spell. If we're going to do this, we'd need to maintain it at all times and come up with ways to circumvent the illusion on a daily basis. There's also Illusion of the Misplaced Castle (ReIm 50) in the core book, but a) it's a ritual, b) it only lasts Sun duration, and c) we'd really need a lab text or casting tablet before Vocis can cast it. I think covenant-scale illusions are off the table as a reactive defense.
Once we get down to Room level, either for the library or the treasury, we have a lot more options. Here are some ideas...
- Illusory ghosts that wander the halls and rooms, moaning or whispering horrible things and giving off a cold aura (CrIm with auditory and haptic species, maybe some Ig thrown in, maybe some Me for the ability to react to stimuli, Group target)
- Frightening noises/voices behind the walls (CrIm with auditory species, possibly Structure target)
- Fill a room with darkness (PeIg or CrIm depending on how we do it, Room target)
- Fill a room with illusory fire that feels hot (CrIm with haptic species, Room target)
- Mask the contents of a room with an illusion of a different room (CrIm, Room target)
- Make the contents of a room invisible (PeIm, Room target)
- Make all the books in the room/tower look identical (MuIm with visual and haptic species, not sure about target)
- Create a room within a room, with an illusion of walls a few feet inside the actual walls, assumes important contents are along the walls (CrIm with haptic species, Room target)
- Audrey's Kaleidoscope from the NWG, which would be very fun with Vocis's spiral sigil (MuIm, probably needs revising)
None of these would need to penetrate, with the exception of those with haptic species. Unfortunately, all of these can be overcome by someone with basic knowledge of Hermetic magic. They're best for fooling mundanes. To affect a magus with illusion-style effects, we'd need to switch to Mentem and add in a bunch of penetration. I have a few ideas, but Vocis can't make them.
- Cause target to forget why he entered a room (PeMe, Touch or Room target)
- Induce fear, guilt, or calm in a target (CrMe, Touch or Room target)
- Panic of the Trembling Heart focused on valuable objects or areas (CrMe, Touch or Room target)