Spun off from the thread "What does it take to participate in the Aegis..."
If a Spell is cast into an Aegis, it is "resisted by the Aegis". Later this is defined as "needs to penetrate a Resistance equal to the level of the Aegis".
It has since the Core Book was written been Errata'ed that Aegis of the Hearth is no exception to wawrds needing to Penetrate. However I can't seem to find a Core Book Errata for this, but it is mentioned in Through the Aegis p137, insert box 'Ferra of Bonisagus', right column. It seems someone had the foresight to mention it in a book.
Maybe it is really the Penetration of the Aegis, and not simply the level which the intruding spell needs? I think not, as it seems the Penetration of for warding effects. However there are examples of comparing Penetration to Penetration, like when casting Muti VIm spells on a non-coperating magus' spells!
Next thing is spells cast inside the Aegis. Here, the Aegis enforces a penalty to casting equal to ½ level. Effectively this reduces Penetration. It is noted that Penetration for creature Powers suffers similar penalties.
What about devices? ArM5 p161, middle column, top says "Effects from enchanted devices are resisted by the Aegis".
But what does that mean? If Penetration is just reduced på ½ level, then it's simple. Devices may then have negative Penetration, but that only matters on targets with MR of 0 or more. One notes that having MR 0 is significantly better than having none at all!
But the same wording is used here - "resisted" as for describing spells cast from outside the Aegis. And here the Penetration needs to be at least equal to the Aegis level. And one could easily argue that this applies for devices as well. It actually never occurred to before it was mentioned in the thread "What does it take to participate in the Aegis..."
Last thing I'll mention is how Aegis protects from creatures with Might. They cannot enter unless they have higher Might than the Aegis level. And since this is a warding-like effect it must also Penetrate. Personally, I like this. I find it odd that a powerful ritual has so easy a time keeping creature out. If you can juuuust pull off casting a huge Aegis, it keeps the big beasts out, even though your Penetration sucks. But you can't actually cast active spells on these creatures, because other effects can't penetrate (well, maybe they can, using different Arts, but still).
By requiring penetration I think you avoid ridiculously high level Aegises (what is the plural of that?), and you have magi participating in Wizard's Communion in order to get a good Pen. I like that. We do that in our current Rhine saga. Come to think of it, I think we've forgotten to track it. It maybe time for a story with an intrusive creature...