Suppose a magus has a circle inlaid into the floor of his lab, and traces his finger along it to cast a ward before summoning something in the middle of it. Can he cast mutiple D:Ring T:Circle spells on the same ring (say, Ward Faerie to keep the being he's summoning bound and Ward Infernal to reduce his risk if he botches), or does he need separate circles for two spells?
Same circle all the way.
In fact i eally like the idea of a magus having a metal circle inlaid in his floor for exactly this kind of thing.
One could even go beyond it and have a "casting chamber" with a large circle to protect against all kinds of interferences so that a magus is nice and safe while conducting rituals, summonings, etc.
The part of this that people casually overlook is exactly that - "drawn at the time of casting". So, having a metal ring inlaid into the ground is not enough - it does present a solid foundation for drawing that circle, but it is not, as itself, "the ring" that the duration is concerned with. (See the last paragraph in that section, p 112.)
Then the question is what exactly "breaks" a ring? Since it requires drawing, does drawing over an existing ring break the older one? Quite possibly. On the other hand, a single physical ring with multiple magical effects drawn one over the other is not a particularly offensive or uncomfortable image.
Alternately, there's nothing to stop one ring from existing within another - tho' only one would be encountered at a time, and all "wards" would not protect each other - the first would be vulnerable to being broken if only a later one stopped the entity/force in question.
Ultimately, there is no definitive answer in the RAW - it's up to the interpretation of the SG and Troupe. Consider the implications, consider the troupe and to what degree they will (mis-/ab-)use it, and go from there.
Elaborate, concentric circles also fit the theme.
There is argument as to whether you can just trace over an existing ring structure.
[quote)Ultimately, there is no definitive answer in the RAW - it's up to the interpretation of the SG and Troupe. Consider the implications, consider the troupe and to what degree they will (mis-/ab-)use it, and go from there.[/quote]
Agreed.
We use the same inlaid circles over and over again. That is not a problem. The problem is when your super-ultra-cool magical circle prison is broken and EVERYTHING inside is released at once.
Concentric circles also work, but we prefer a single all-encompassing circle. It is a saga thing here, some prefer one, some the other 
Cheers,
Xavi
Also (lazy ass parma - my books are on the shelf and I'm to lazy to get up and check them), I believe that the sample lab plan in Covenants has a "spell circle" in it, which adds weight to the "reuse inlaid circle" argument.
The downside of such circles is that it makes harder to break the circle even if you want to. PeVi PeVi Pevi.
Plus think of the intimidation factor.... walking up to your target Magus and finding him surrounded by six rings of something. 
"Better leave him alone... he's paranoid."
Just curious - what does "breaking a circle" require, in diff troupes' Sagas?
If a person leaves the circle, they have "broken it, in one sense.
Another interpretation is that if a mundane item falls across the circle, it has been broken. A leaf, a string, a foot scuff, anything that is not warded, potentially.
Magically drawing does not necessarily mean it must be magically erased for a ring to be "broken" - possibly, but not necessarily.
What are the diff thoughts here?
I require physical breaking of the circle. If it is drawn in the dirt, someone blowing hard or throwing a stone that lands in the perimeter (and scrambles the sand/dirt) can break it. If it is embedded in the stone or floor it would require quite some effort to break it.
According to RAW a single leaf would break it. At the same time there is the example of the circle of demon trapping in the Normandy tribunal, that is constantly washed over by the waves of the sea and not broken. 
So, whatever floats your boat, as usual IMS it requires it to be phisically broken. Simple superposition of stuff on the perimeter will not destroy it.
Cheers,
Xavi
So you're ignoring the RAW?
Those describe the "circle" as what is drawn at the time of the casting, not what it is drawn upon. A magus can have a floor with a metalic inlay, one pace thick, made of pure unbreakablomite... but then they still have to draw their "circle" on that.
If drawn with chalk, or in the dirt, it's fairly vulnerable. If drawn with "magic" (as the rules suggest is possible), then it has other qualities. If scratched in stone, or in metal, then yes, that is harder to "break" than mere chalk, but there are ways other than destroying the stone or metal.
The "circle" is part of the spell, drawn with the casting - it is far more abstract than something that the circle is drawn upon, even if that could also be called a "circle".
Really? I don’t get this impression. The RAW simply say that the magus must trace the circle by walking its circumference, not that he need actually physically draw another circle on it.
I’ve always taken this to mean that such a circle remains “unbroken†as long as the band remains whole and unmoved.
The RAW do imply that a Circle is a part of the spell that must be walked by the magus, yes. But the passage above also implies that a pre-existing circle can be designated as a Circle target simply walking along its circumference during the casting of the spell. Otherwise, what would be the point of having the inlay (and the entire passage above) at all?
No, respectfully, they really don't. I believe you're (incorrectly) inferring most all of this, tho' I can see how one could easily misread this section, especially if you go in with pre-conceptions. That doesn't mean it's not a valid houserule, quite the opposite - but that's all it is.
Actually, the rules imply no such thing, and in fact specify exactly the opposite, on both counts...
(emphasis added)
So, the basic concept is that it must be drawn - as stated twice, quite clearly. Even magically, the spell must have a range of Touch - he's "drawing" it with magic, either with his finger or focus.
Of course, but the tracing is what is important, not the band. The band is not (necessarily) designated as anything - it merely serves as a prepared surface for the drawing of that ring. It is the difference between drawing on a chalk-board and on laid flagstones, or whatever.
What an existing band does is to speed up the process, not (necessarily) aid in the unbreakability of the circle. The "walking" leads in to the issue of speed, not an escape clause from that drawing. The phrase "or until the ring is physically broken" directly refers to the ring which is traced, not (necessarily) the band that is laid into the ground.
While we, here in these forums, are informal with the interchangeability of the terms "ring", "circle" (which is not used in the this section) and "band", that section of the rules is not. The ring must be drawn, the band may pre-exist and aids with the drawing of the ring.
The sole exception is what is probably causing the confusion. Because they say "The ring may already exist...", it creates a contradiction - the ring must be drawn, but the ring already exists? But I take that in context, not isolated - the point of that short paragraph refers to the speed of the drawing. My reading of that is "the location of the ring may already be pre-defined" - it is not the ring, but will serve as the prepared medium upon which it is more easily and reliably drawn, nothing more. I base this interpretation off the emphasis in every other part of the rules on the actual drawing, and the fact that no additional emphasis or clarification is given to what would otherwise be an important exception to that drawing process.
If you want to interpret it the other way, that this contradictory passage negates the repeated mandate that the ring must be drawn at the time of casting - hey, your game, make yourself happy, all good. In my initial post this is exactly what I was asking - how many rework the RAW to suit.
(btw - I know I may be dead-ass wrong on what the author's intentions were - but I also am confident that, given the language used, that's how it reads.)
Yup, this is basically the long and short of it - a difference in our interpretation of that exception. Put another way, I believe we disagree about the meaning of the word "trace". I take it to mean "walk around" on the basis the second paragraph is entirely unnecessary if the intention of the rules was to have you physically draw a circle every time (then the first paragraph would be sufficient in all cases and it really would be a house rule). If I understand your argument, you take it to mean "physically draw" because the first paragraph says that the circle must be "actually drawn" while the spell is being cast. That may be, but it's noteworthy that even the word "drawn" is ambiguous. I can arguably 'draw' a circle with my finger without making any physical mark - if the mark is already present as a pre-existing circle, only the 'drawing' is necessary because the boundaries of the circle are clear.
I don't even think of it as a house rule, because I don't believe I'm going counter to the intention of the text - but YMMV.
Actually yes.I (and my troupe) completely ignore the RAW. I and my troupe think that the rules for circles as written are quite bad, so we prefer our own version. Easy
You have to pace around the circle at the time of casting all right, but you have a preset circle that will make it much more durable. And we also ignore that crap about a hair across the stone circle breaking it since we feel it is the least magical stuff we read in a while. YMMV. Our milleage varies a lot from the RAW in quite a few instances. This is far from the most extreme instance
Cheers,
Xavi
But the RAW supports a (semi) permanent circle... Covenants p. 104.
"Defacing the circle can destroy the ring and circle wards, so durable circles are carved into stone, or made of metal fastened to other surfaces."
Its in the section on Sanctums, under the heading 'Ring and Circle Wards'...
If a magi were to carve a Circular "groove" in a stone floor. And then "draw" a ring by inlaying that groove partially with a thin layer of a metal, let's say copper while casting a ring duration spell, then repeat with a different metal, say silver and cast a different ring duration spell... would stacking the metal rings within the grooves "break" the ring as they are still intact, merely stacked?
If you could do this could you not create a single circular/ring ward/spell with multiple effects?
If you're doing that, you're definitely moving beyond the rules as written and creating houserules - define them as you want.
Whilst this is to an extent true, your abstract definition of drawing a circle would allow a magus to inscribe a circle in mid-air. I prefer to think of the physical circle as an anchor for the spell, much as the runes of rune magic, and that that is why a permanent stone or metal circle if favourable - it makes breaking the anchor more difficult. That the circle need be traced when casting the spell is thus the defining of the anchor for that specific spell. A circle in chalk or sand is easy to create anywhere, but trivially destroyed.
As for the OP's question, there is to my mind so reason why you cannot have a circle support multiple spells. The wisdom of doing so is a little questionable, however, since a mishap would destroy the lot of them and whatever you were sustaining or containing would be free to continue its normal path.
I see no reason why one could not (magically) draw a ring in mid-air, tho', as you point out, the wisdom of doing so may come in to question.
You may very well have the right of it, especially when you realize that such a mishap can happen while you are tracing over the first circle to cast the additional spells! If you don't have a permanent circle readily available, you might want to nest them rather than tie all the spells to a single circle.