Here are all the current proposed clarifications and revisions for container targets. There is a question at the end.
Spells with "container" Targets, including Circle, Room, Structure, and Boundary, can work in one of two ways.
First, they can affect any valid target within the Target container at the time of casting, and continue to affect those targets even if they leave the original target area, and even if the target container ceases to exist, for as long as the spell lasts. It does not affect anything that enters the target container later, even if the spell is still in effect.
Second, they can affect any valid target within the Target container during the spell's period of effect. In this case, a valid target that leaves the container ceases to be affected by the spell, and a valid target that enters (or re-enters) the container is affected, until it leaves or the spell expires. In this case, the spell ends if the container ceases to exist, even if it has duration left.
The way that a particular spell works is fixed when it is designed, and cannot be changed by the casting magus, although a spell working in one way is similar to a spell that is identical apart from working in the other, and so knowledge of one gives a bonus to inventing the other.
For example, a spell to put pink dots on people's foreheads with Target: Room and Duration: Moon could work in two ways. In the version that works in the first way, everyone in the room (on whom the spell Penetrates) at the time of casting gets a pink dot on their forehead, and this pink dot remains on their forehead until the new moon and full moon have both set, no matter what happens to the room. In the version that works in the second way, anyone who is inside the room (on whom the spell Penetrates) until the new moon and full moon have both set gets a pink dot on their forehead, even if they were not in the room when the spell was cast. When they leave the room, the pink dot disappears, although it reappears if they go back into the room. If the room ceases to exist before the duration expires, the spell ends early. These two versions are two different spells, and a maga who wants to cast both (and cannot reliably cast them spontaneously) needs to create two spells.
Being Inside a Container
The standard Ars Magica rules use an intuitive definition of "inside" when determining whether something is within a container target. A person standing in the centre of a 1 pace diameter circle is inside the circle; someone who happens to be standing directly above it three floors higher is not. A person standing on the battlements of a castle is within the Structure, but a crow flying over the walls is not. If your troupe needs a more precise definition, you can work something out, and nothing should break.
Persistence of Containers
The standard Ars Magica rules use an intuitive definition of whether a container still exists to be the target of a spell. If the same container still exists, then the target still exists. An island does not cease to exist when the tide rises or falls, unless a causeway joins it to the mainland at low tide. A room does not cease to exist when the door is opened, but a room inside a tent does cease to exist when the tent is packed away. Containers can move if they are still the same thing while, and after, moving. Note that certain choices of container make spells vulnerable: if a Room is just a tent, the spell can be ended by collapsing the tent, while if a Boundary is just a fence, the spell can be ended by pulling down any part of the fence. Containers can get larger or smaller, but spells do end if the container becomes too big for the spell to affect.
Circles and Boundaries require some extra comment. A circle is a different circle if its size changes, or if it is transferred to a different surface. Circles are very vulnerable, which offsets the power of Ring Duration and Circle Target. If a Boundary is nothing more than the thing marking the border, then it is as vulnerable as a circle, and cannot be moved, altered, or expanded. If a Boundary is something like "the waterline of this island" or "the wall around this covenant", then it is much less vulnerable, and may change with natural changes in the thing.
Here's the question. The above clarifications change the rules for Circle. Under this proposal, type 1 Circle Target spells are not ended if the circle is broken. I don't think this change will require any changes elsewhere in ArM5, although I could be wrong, and I am inclined to think that it is worth making the change to maximise the consistency with which different targets are handled. It would require a small change to the description of the Circle Target. Comments?