I believe RAW examples imply that the ring has to be physically shattered. A circle of salt smudged, a brass ring broken - that sort of thing. Otherwise, some canon examples make no sense. I think all canon examples are visible, though - so perhaps covering a circle up breaks it?
For my money, while a "leaf falls over the circle" is a nice cliche, this makes circles far too vulnerable to be practical for nearly anything. I\m in favor of the "substantially compromised" idea - when enough of the circle is covered up or otherwise deformed, it is broken.
I think this would make such effects too flexible. I'd again say that the boundary ceases to be the boundary the spell was cast on, and the spell breaks, when it is "substantially changed". Adding a new tower to the wall won't do it, but pushing the wall back 10 paces might and breaching it certainly will.
That just soprings to mind the idea of the covenant wanting to grow, so they build new buildings OUTSIDE the aegis. They will ibnclude them in their new aegis ritual next year. No problem. Now, that pesky demon gets into one of the new buildings and manages to avoid detection. BAM! Into the eagis in the next winter solstice.
This already exists in the Rhine tribunal with Durenmar, but I never thought about using it IMS.
I could see a Circle of Warding on a solid surface be mobile, like on the bed of a wagon or the deck of a ship. FLipping around the table with Ward Against Cute Puppies? Errrh, aren't Warding Circles actually described to be domes? Or is that just a cosmetic spell description? I'd say the pups stay inside the dome.
Wards on moving metal hoops? Perhaps this explains why 17th century women were able to fend off their suitors, the hoops in the dersses had multiple layers of wards. No sorry, that dog won't bark for me.
As for the chain, I can see it work once it is laid down and conncted at the ends. When it is just a long chain it's not active IMHO. And not when moving around as a chain-circle either.
Actually, the columbae (exmiscellanea ward specialists) do create mobile circular wards. The robes of dusky dawn (of whatever they are called) are a Columbae item that basically does that: the outer rim of the robe has dozens of potent wards with circle duration IIRC. So yes, a circular ward can move
IMS CrIg Circle/ring torches/fires are really common, and others as well: ReAq circle/ring hole in the kitchen that draws water from the underground, circle/ring PeTe to destroy waste thrown inside.... The list would be quite large if we bothered to compile them all. Those are effects that we have developed in a lot of sagas. Right now we simply assume that any covenant would have them active all around. We just dedicate 300 spell levels to that kind of effects in casting tablets as default when we build a covenant.
The Columbae, and their Minor Hermetic Virtue "Circle/Ring Magic" allow them to do several things with Ring/Circle magic that others cannot - or do similar things quicker/better than standard hermetic circle/ring wards.
I'd not necessarily look to them for an definitive idea of how standard hermetic ring/circle wards work, as within their narrow field, they are superior.