Here is a weird thought.
Say you want to mess with another Covenant, so you put up an Aegis of the Hearth in the way of their own AotH.
Say your mage is capable of casting a level 20 AotH while invisible and silent.
You wait in hiding for the target Covenant start their own level 30 AotH and your mage starts casting the level 20 on the same Boundary.
The level 20 is finished first.
What happens?
First your magus needs to overcome the Covenant's old Aegis, provided that he has no valid invitation to it. If he succeeds, the Covenant's magi need to overcome your's, as you didn't invite them.
Sort out who is when inside or outside of which Aegis during the operation, as the ways to overcome the Aegides depend on it.
If the Aegides are cast successfully, they will interfer with each other while they last.
A Quaesitor investigates and you may be Marched.
Or, you are Wizard Warred by the whole other covenant when they figure it out.
Yeah. I think preventing magis from using their magic in their own home sounds like Deprivation of Power to me.
The active Aegis with the highest magnitude will suppress the lower one.
And typically, the recasting is timed to take effect at the same time as the old one goes down in order to not leave a gap so the fact that the lower magnitude Aegis is cast faster does not really matter that much. Also not that you also need to run the boundary which makes staying hidden hard depending on the layout.
For the plan to work you'll need to:
1 - Have an aegis higher than the existing one
2 - Have the ability to cast it faster than the new one (in order to benefit from all the magi being too busy to notice the rise of a foreign Aegis)
3 - Stay un un-noticed while casting your Aegis and walking the boundary.
Personally, it seem just easier to PeVi the Aegis and add another PeVi Room where you will be attacking that has the same benefits as the Aegis vs casting totals, to trigger within a watching ward. Or better you, make it a PeVi Parma.. or both... & then enter the room and boom. Or don't enter and just have another watching ward with a CrIg to "clean" the room and have a grog report back that the room as been "cleaned"
W
No, you don't.
Boundary spells, including the Aegis, do not work like Circle or Ring spells.
Participants walking the boundary of the Aegis is a custom, and the reference to this custom is not included in the Definitive Edition text of the spell.
Please provide the quote from ArM5.
Corebook p.161"The entire covenant usually participates in the Aegis ritual, which often
ends with the participants walking around the perimeter of the protected area in order to define it."
Still valid until DE is officially out.
Same place as other quote: "The Aegis is also able to block foreign Intellego spells, even if they cannot normally be blocked by a Parma Magica, and spells that were cast before they entered the Aegis, such as an invisibility spell cast on a magus outside the Aegis"
Most folks interpret "block" as suppressed as Aegis is a Rego effect primarily. Some say it nullifies it.
If you are able to participate in both the old and new Aegis, then maybe the two can coexist.
W
A recently cast ritual would be an arcane connection to the caster, would it not?
If so, that would make this a spectacularly risky plan.
The words "which often ends" do not, in fact, mean the same thing as "which must end". Walking the perimeter is, by the very words you yourself quoted, not a required part of creating an Aegis.
My pointing out the deletion of the reference in DE was simply to emphasize that, in fact, there's no way to try to rescue this by appealing to "intent". The intent is clearly that the walking isn't necessary, or else it would be preserved in DE.
And that it is simply a tradition associated with the spell, not part of the spell itself, is further emphasized by HoH:Societates p.16: "many of the traditions associated with it include Mercurian practices, such as everyone walking the boundary together."
So. The original wording, understood literally, is that the walking isn't a necessity. The DE deletion of any reference to the walking makes it clear the line editor thinks the walking can be omitted. HoH:S is explicit that that the walking is a tradition.
Requiring that the perimeter of the Aegis be walked for the spell to work is accordingly purely a house rule.
Has anyone factored in that different level rituals take different times to cast?
If one gets cast before the other completes?
This is now (ArMDE 09-spells):
Any foreign supernatural effect that originates from outside the Aegis and would affect anything inside the Aegis must Penetrate a Magic Resistance equal to the level of the Aegis of the Hearth to take effect. If it fails to Penetrate, the effect fails completely and harmlessly. This applies to all effects, including Intellego effects that would not normally be blocked by Magic Resistance, and effects that would move something across the boundary of the Aegis.
So it is not just comparing magnitudes, but the Aegis cast later from outside needs to Penetrate a Magic Resistance equal to the level of the Aegis already in place. If it does, it is successfuly cast and coexists with preexisting Aegis.
For casting a new Aegis from the inside of an existing one and without invitiation we have now:
Any foreign supernatural effect originating within the boundary of the Aegis must subtract half the level of the Aegis from any Casting Total or equivalent.
This allows the second Aegis to coexist with the first but with reduced or zero Penetration - provided the second caster can prevail against the first Aegis and still cast his Aegis successfully. He might suffer big time Fatigue though, and his Invisibility magic was also under attack from the Aegis into which he had snuck.
Insisting on overridden text from the old core book on complex issues is not much helpful for arguments on the Forum now - but might still hold in your sagas, of course.
I just answered your call to quote from Arm5. Hardly insisting...
I agree. Officially, the not fully integrated Aegis that is based on Mercurian rituals, cast with ceremonial casting, often alongside another Mercurian "ritual" communion for a Boundary effect, does not explicitly require walking the boundary. I guess you could do the ritual in the basement of the Covenant without having any idea if the boundary your are trying to affect is still intact.
I wish the rules described what ceremonial casting entails for boundary effects. Saying tradition often has folks walking the boundary is not much to go on. Is it enough to lit a candle in the basement?
I also think that such a Covevant is playing a risky business as even natural events may make the ritual fizzle such as forest fire, flooding, landslide, avalange, fallen wall, if it breaks the boundary. Not even getting into active disruptions by a rival/enemy. Traditions exists for a reason and the Aegis, being cast every year, using vis, in a communion, having multiple Magi participate with all their flaws, is risky business at the best of times. It is reasonable that Magi seek to minimize the risks as much as possible and if walking the Boundary helps knowing what you are affecting, then they would do it. Also, in DE, it is explicit that the Aegis needs extra magnitudes for size increments meaning that if your boundary changes and becomes bigger, you need a higher level Aegis which means that Aegis are many which means that mastery Summae/Tractati are less common then they used to be especially if your boundary is vast. Keep in mind that the "Basic" Aegis is 100 pace wide which is about 2 acres size or a mid size castle. Such a small boundary is probably man made. Man made is often easily disturbed...
As SG, I might be tempted to add a botch die if the boundary has been tampered with and the caster does not know about it.
Corebook p.113 "Boundary: The spell affects everything within a well-defined natural or man-made boundary. This could be the wall of a city, the edge of a village, the shores of a lake, the edge of a forest, or the bottom of a mountain. Since the ocean is not obviously bounded, it cannot be affected in this way. As with Room and Structure, there must actually be a boundary for this target to be useful. It cannot be used to simply affect a really big area. A spell with this target must be a ritual."
W
Ceremonial casting/ritual casting is no different for a boundary target than for any other target.
What suffices for T:Ind should suffice for T:Boundary. Exactly how the casting is done is not defined - probably on purpose.
I have this image of invaders besieging a covenant by casting a larger area aegis outside the existing one, such that (assuming it fails to penetrate) spells cast from within the first aegis would fizzle reaching the second, essentially confining the magic of that covenant to the actual grounds of the covenant.
And now we are back to medieval siege warfare.
I like the visuals but since both Aegis are dispelled with simple non-ritual spell, the likeliness of a magical medieval siege is almost null. Unless you count the siege in rounds...
W
True, I guess the ceremonial actions can range from nothing other than slow motion formulaic casting to having hours long dances with specific astrological references and artifices.
I think Arm5 could have done more with Ceremonial magic. Could have been a good way to better define the Mercurian Magic Magi. They get to pay half the price of Vis but the also are bound to do some archaic Mercurian special dance moves 
W
Iād prefer if Boundary worked like Circle/Ring and explicitly had to be walked as part of the casting.
First not all SGs accept the idea that an aegis is so easily dismissed, which seems to fly in the face of the purpose of the aegis.
However, even if you do accept it keep in mind that he group inside both aegis have to penetrate the outer aegis to target it from the inside. On the other hand the outer group can stand between the two aegis and cast freely to bring in the inner aegis, and then have their aegis in full effect throughout the covenant.
But it would be odd for Boundary to be different from Room and Structure in that respect.
I think I agree with WilliamEx that it's not good to cast the Aegis from a cellar. I feel that some part of the boundary-as-edge needs to be perceptible by the caster, it shouldn't be enough to perceive some element of its interior.