The problem with handling it in terms of a maneuver is that you only get one free compel against the critter's supernatural high-concept... and then you're pretty well boned. While this can represent the "circles can fail" mentioned above, I don't think it does so well, or fairly... especially in circumstances where there are several attacks against the circle at once from multiple weak sources, as opposed to a single massive blow from an arch-demon.
Not exactly. You only get one free tag to represent a momentary advantage, but if it's a scene aspect, it stays on the scene and can have impact through GM compels. In this case, the scene aspect represents a "game fact" that the GM should take into acount via compels as appropriate.
For example, let's say I performed a maneuver to start a fire in a field of dry grass. This would set up a scene aspect "Brush Fire!", and this aspect's effects would
not be limited to a single free tag, after which the fire has no effect. The originator would get a free tag to reflect a momentary advantage, but the aspect would stay on the scene, potentially generating GM compels against everyone present as the fire began to spread out of control. Appropriate compels might be fear-based reflecting self-preservation urges to avoid the fire, and environmental damage might well be appropriate for anyone who chooses to stay in the burning field. All of this for the cost of a maneuver and a match or lighter; no Fate cost at all.
I would model circles similarly. Basically, you use an action to draw a circle, conting this as either a maneuver or as justification for a declaration to create the scene aspect "Magical Circle Drawn Around Us" or something like that. The aspect now represents the fact that there is a circle protecting those inside it from entities that are affected by circles. At this point, the protection of the circle has little to do with the aspect, instead it is a function of the high concept of those creatures who by nature have issues crossing magical boundaries. So you don't need to tag the aspect to ward off, for example, a ghost -- the GM should be expected to compel the ghost's high concept to reflect that it can't cross the circle. The ghost might move on, looking for easier prey, or it could choose to lurk nearby until the circle weakened (in about a scene's worth of time). Non-supernatural creatures would be entirely unaffected by the circle, because their high concepts do not include any such weakness. They could also break the circle. Creatures in between -- with more substance but enough supernatural nature to be repelled by circles -- could either accept the compel and be held at bay or flee, or might be able to buy off the compel. (If the creator of the circle had been concentrating on holding the circle, it might not be unreasonable to rule that this counts as a (standard, not evocation-style) block, based on the creator's Lore. Until the block breaks, those in the circle would be aided by the block against affected creatures that attacked them. (Alternatively, Conviction or Discipline might be allowed instead of Lore.)
What gives someone this capability?
* One option is to treat this as a common ability costing nothing to do -- ie, you're just drawing a circle, and if your maneuver/declaration roll works, then the circle has power over creatures that don't deal well with such things.
* You could also rule that the circle on its own is just chalk, and that powering it requires 'will' in the form of 1 mental stress (like casting a spell).
* Or you could treat this as a case of 'borrowing' a lesser version of Thaumaturgy (call it "Minor Ritual [-1]" granting access to a single ritual -- in this case a Thaumaturgical maneuver creating the scene aspect above). In this case, you'd spend a Fate point to borrow the power, then use it to cast the spell creating the circle.
* Or it's a stunt (allow people to spend a Fate point to borrow it) that adds a trapping to Lore, allowing it to make magic circles via declaration (possibly requiring mental stress as well).
I think that any of these are 'valid', its up to the group to decide which feels best. I think I like the last option, which (when simplified) means that you spend a Fate point, mark off a mental stress box, and make a Lore declaration, then the GM compels creatures that should be compelled. (If you fail the declaration, you can keep trying at the cost of additional mental stress; the Fate point already spent grants access to the 'stunt' for the scene.) I'd also probably allow Conviction or Discipline to be used instead of Lore. As an option, characters could actually buy the stunt (and not need to spend the Fate point to borrow it), but I wouldn't expect this to be common.
