I can see a book with 2 linked spells and a number of utility spells to use with them.
1: create a perfect circle. Size at the caster's desire
2: make the circle unbreakable by mundane means: repel stuff that could damage it.
3: spells like mystic tower, mystic grannary, mystic fish pond, mystic fire, mystic waste disposal, mystic well,.... all with circle/ring parameters 
Cheers,
Xavi
The circle does not have to be perfect, although that is certainly more aesthetically pleasing.
That's the part that is easier said than done. You have to repel a lot of Forms to protect your Circle... And I am not sure that the same circle can be used for these spells: the target has to be inside the circle, and I don't think the circle is inside itself 
Why not ? As long as none of your enemies is good at PeVi
Except that for Creo is that which is created, NOT that which is effected by the thing creates.
People can walk in and out of the fire all day, so long as the circle is not broken, and get all kinds of crispified.
As for the others, the idea would be to get the targets inside a space and then use the spell on the outside. For example, a (covenant owned) town at night. Call to Slumber the guards, and then... LEPER COLONY!!! :mrgreen:
Or a circular version of call to slumber to keep someone asleep (great on prisoners)
IMS, though, Circle/Ring spells continue to effect any target which enters a circle after its been drawn, and until it is broken, if its a spell which only effects those things which remain within it. That's a house rule though.
Oh, i was wrong about test in the flame.
The test in the flames won't work, but because it's a creo spell creating something with a target which is not individual or group.
Wouldn't be the first contradiction in the rules. I think the broader vision of circle magic is more fun, and that's usually the best way to determine whether to use a rule or not.
Another question I had about this: Assuming a Creo Aquam, either a Circle/Ring or a a Duration: diameter. When the effect expired (or you stepped out of or broke the ring) would everything wetted by the spell become dry, or is "wetness" a property which become proper to the effected thing, in Ars Magica physics?
But circle target can't allow anything to be created since it's defined as the content of a circle. A circle being a 2 dimensional things, it can't exist.
Would circle spell creo create a unlimited in high pilar of flame? what under the earth? What if you use said circle of created flames on your buclar and target with your new weapon the lord castle, using it as a laser?
I prefer to stick with defintions which answer the problems.
The same problems exist for Wards, and yet we live. Can a Dragon fly a mile above a ward v. dragons and drop a rock on the protected person? Tough call.
You can't make mobile circles in the way you describe, I don't think. The ability to make mobile Circle/Ring effects is a Supernatural Ability (Craft Amulets) in ROP:D.
I think that the sphere of influence for Creo and for Warding in cricles is situational. Generally, I believe that Wards ward in a sphere with the Circle or the tallest thing contained therein as the circumphrence.
I'd say ditto for max effect of a Creo (Form) size, too, though some things, like Fire, might have size determined by intensity and power of the spell.
But I like Test of Flame, and Circle-Creo in general. They add flavour and allow for some truly fun applications without overdoing the power-builder thing.
In Hedge Magic, witches are prevented from flying over an Aegis of the Hearth at any height, so it's very possible the answer is no.
The question of what 3-dimensional extent Circle wards protect has come up before. But since then, I noticed something in the RAW that weighs in on the side of "a sphere":
That being said, I like the VYOM* paradigm. For example I definitely see an indoors circle as generating a column of warded space (if the ceiling isn't stupid high).
Agreed. As the Romans said "usque ad sidera, usque ad infera" ("all the way up to the stars, all the way down to the underworld" - that's how far "vertically" you were considered the owner of your land by Roman law).
Do you have a reference for that? The only thing similar I can find is on page 7, under "Hedge Wizards and the Aegis of the Hearth", and that only mentions that they cannot fly through an Aegis.
Nope, you're right. I made it up whole cloth. Oops.
Here's another question:
Magic spell rules in Ars Magica seem to be quite loose and variable, often contradicting set guidelines. For example, there's the distinction between sustained and immediate effects:
Example of Immediate: A MuIm effect that changes all liquid in a room into putrid tasting goop. If the liquid leaves the room, it remains icky. It returns to normal when the spell ends.
Examples of Sustained: Notes of Delightful Sound - MuIm - all music played in an area for the duration of the spell sounds more beautiful. If new musicians are brought in, their music is likewise effected. If musicians leave, their music becomes unaffected.
Lungs of the Fish: MuAq(Au) changes all water which enters the (mouth? lungs?) into air for the duration of the spell (often Sun), allowing one to breath underwater.
These are all possible with the same guidelines.
Variants of the Immediate/Sustaining divide apply for Structure, Individual, Group and Room... even Boundary.
So my question is: Can this apply for Circles?
Immediate Circle: Circle of Slumber -- All those in the circle fall to sleep, and remain asleep for as long as the Ring remains. They may be moved to new locations (hidden) outside the ring, if that is desirable. New persons entering the Circle are unaffected.
Sustained Circle: Circle of Slumber -- All those in the circle fall to sleep, and remain asleep for as long as the Ring remains and they remain in the Ring. If they are removed from the Ring, the spell ceases to effect them. If someone new enters the ring, it does effect them.
Do people have a preffered version, or are both acceptable?
I would have no problems with either spell. They would be 2 separate spells, though, not a single spell with a variable dial.
Xavi
IMO, sustained circle don't respect the circle parameter. I know some canon spell do not respect, but... i try to avoid them as plague. But otherwise than that argument, it's fine.
Perhaps there should be a minor virtue: Sustaining Magic. "Allows magi to design spells that continue to effect new objects which fall into a target area for the duration of the spell, though they cease to be effected when they leave the target area."
On the plus side to the non-sustaining school of magic, it means that "circular ward against fire" actually protects all people in the circle at casting time from fire wherever they go, until the ring is broken 
Funnnily, I think that the sustaining version is more balanced than the RAW version.
I prefer to run it that way too.
But Circle RAW can be read as either "within ... at the time of casting" or "drawn ... at the time of casting." The problem is that Ring RAW can only be read as "drawn at the time of casting." This makes the "drawn" interpretation stronger and more likely to be valid.
Therefore, sustained Circle is probably RAW.
Hmm I've never understood where that view comes from.
Can it though? Reading the parameter I see the phrase is written
I've always seen "at the time of casting" as a description of the drawing of the circle. It tells you that the Magus has to draw the circle when he casts the spell not before. Reading the phrase so that the "at the time of casting" instead limits when the targets are "within" said ring would mean that the circle could be drawn before you cast the spell right?
If the phrase read "The spell affects everything within a ring drawn by the Magus with white chalk" would you assume that the ring has to be drawn with white chalk and/or all the targets have to be holding white chalk?
That phrase to me reads exactly the same as similar phrases that start the descriptions of room, structure, and boundary.
Even reading further I just don't see any part of the circle parameter that makes it different from other parameters which can be sustained or not sustained. If it's just personal taste okay but I see a lot of people who seem to feel that the rules for circle and/or ring disallow sustained type effects even if other parameters don't.
It is a game about magical arts. If it is cool, go for it. Both versions of the spell would work.
If you feel better, consider someone or something crossing the circle boundary as "breaking the circle". In fact this is RAW IIRC
Xavi
Maine and your too long post>
IMO, yes you could prepare circle (engrave on your room for example) and allow item or even waiting spell to take effect on those (otherwise it would fail since no item can draw a circle without a specific effect), so I read "at the time of the casting" related to "affect everything within".
Another reason why I think it can't work (as for room and all other parameters, except wards which are explicit counterexamples, and the aegis which is a breakthrough not respecting the rules), is the limit of the arcane connection: you must be aware of a target to affect it.
When you cast, for example, a ReME spell on a castle, to make everyone asleep at moon duration and after you have left, a novice called Sebastian enter the castle, you were not aware of his presence, thus he can't be affected. And the only way to achieve it is to say, that it does only affect what you aware of, that mean "at the time of the casting", since you are not necesseraly aware of what will come after.
I think those special things are fine and provide a nice supernatural feeling. ("the castle where everyone entering fall asleep"). However, when it comes to circle, I'm more cautious : if someone is affected at the time he enters the circle, is on his boundary. If the circle is a physical boundary, all future people affected will be affected at the edge. if this is some kind of "petrification" effect, i find it really bad to have statues on the edge of the circle... without even considering that they will effectively broke the circle, freeing everyone .
But if you think the circle is a metaphysical boundary, I could read that everyone entering the circle break the effect: they weren't part of it. There is a myth (I don't remember the name) where one enter a circle, and the circle is broken at that time, freeing every one who has been kept prisoner for years, isn't it?
In front of the problems a general saying "that could work" creates in regard to particuliar effect, I tend to stick with "limit of the arcane connection + description of target". It's easier for the players, which otherwise have to guess what you will say.
But I don't deny that some of those effects are really cool!
IMS this is exactly the kind of spells you can have with experimentation discovery or modified effect, for example.