So, we are getting ready to start an Ars Campaign and the GM has placed our covenant inside a mountain, specifically in a series of caves inside Mount Etna. For a few weeks now, I've been trying to think of how Aegis of the Hearth is going to work in this instance. In a normal Aegis ritual you "walk the perimeter of the covenant to define it". How in the heck are we going to walk the perimeter of a covenant that's inside a mountain? Walking around the whole darn mountain would take days. Maybe I'm over-complicating this, but it does seem tricky.
The boundary doesn't need to be a natural one. So assuming you are able to mark out the boundary inside the mountain somehow, thereby defining a threshold of sorts, you could use that. I could quite see the magi plotting a large circle centred on the middle of the covenant and then marking out every tunnel and pathway that intersects that circle. Perhaps walking the boundary is a misnomer in this case. Perhaps it is more a case of visiting each tunnel that crosses the boundary.
Alternatively, throw in some additional magnitudes for size and you could feasibly circle a given path around a mountain. Of course, attempting to circle the mountain before the ritual is finished is likely to become part of covenant tradition. I can't remember how necessary it is to mark out the boundary (as you do with a circle). If it's mandatory then you'll need some means of ensuring that your magi can encircle the mountain quickly. Perhaps the gleaming eagles that nest within the volcano can be bargained with to carry the magi. Or perhaps the spirits of smoke and steam that seethe and multiply in the mountain can be bound into service.
The Domus Magna Criamon is actually a complex of caves.
I don't know how they cast their Aegis, but I'm pretty sure they have one.
The text actually reads: "which often ends with the participants walking around the perimeter of the protected area in order to define it." I would think a simple model of the covenant would suffice, or the casting magus could fly around the outside or none of the above.
The target of the spell is boundary, which means a well defined area. The range is touch, which means the casting magus must touch the boundary, IMO. So simply touching the side of the mountain should suffice. If you are still interested in the "parade", you could have all the people involved in the Aegis take a stroll through the inside of the covenant, spreading flower petals or holly leaves, grains of wheat, or some such.
Or perhaps using a map of the covenant to "define" it? Maybe the casting magus holds the map while touching a wall of the covenant or something?
Perhaps they walk the outermost wall inside the mountain/cave.