Magic Circle

...as with the Haldenseed theorem, many of our most basic principles were derived by Magisters of White, as it is said that mathematics are as critical to their craft as the actual practice of magic. Magic circles, which must be crafted with utmost precision and whose function depends on their geometric organization, are but one example...

From a Mordian book describing the history of mathematics.

Overview

The 'Magic Circle' effect is one of the most fundamental in all spellcraft, with dozens of varieties, and is necessary for the effective use of many other types of magic. This spell creates a one-way magical barrier, which bars all magical effects (though powerful effects can potentially push through). It can either be inward focused - preventing magic from exiting - or outward focused, preventing magic from entering. It has no effect on matter or non-magical energy. A Magical Circle can not only block ambient mana and magical effects (i.e., spells), it can also block Transcendent and other Spirit Beings (as they are comprised of magical ectoplasm) and even disrupt the Dweomers of magicians who pass through. Though it only functions one way, if discrete objects - from material items, intense energy, or spells - pass through in either direction, then the magic circle is disrupted.

Application

The 'Magic Circle' is incredibly useful, creating a nigh-impenetrable barrier against all forms of mana, from spells to ambient mana to [Spirit Beings]. It can provide security against long range spells (such as sympathetic targeted spells) or even serve as a trap for unwary spellcasters or a defense in a duel, but its primary uses are utility. It can prevent interference for testing or casting of particularly sensitive spells, hold a concentrated or otherwise peculiar mana environment securely, and, perhaps most famously, serve as a summoning circle. In this role it not only holds the summoned entity, but also provides a locus for the magic.
More complex arrangement can serve as 'external dweomers' or efficiently concentrate the efforts of multiple magicians into a single spell. The options are both crucial and diverse, and it is near certain the full slate of possibilities is not yet known.
It should be noted that no magic circle is infinitely powerful, and while

Method of Operation

A Magic Circle effect works by forming an sub-millimeter thin, intense layer of mana that comprises the circle itself.  One facing, the 'transcircular' face is almost infinitely flat and smooth, while the opposite 'ciscircular' face forms a gradient.  The mana cannot flow across the disconformity of the transcircular face, forming an asymmetrical 'front' that contains mana to one side, while permitting the flow of small amounts of mana from the ciscircular face, and light and matter from either side.
Magical entities - spells, spirits, etc, pushing against the transcircular face are opposed with equal force exerted on the face.  This causes the circle to deform, but only slightly - so long as the energy of the entire circle is not overcome, the surface remains smooth and unbreached.  Typically, this leads to the entity being pushed back or even collapsing.
However, if something manages to breach the barrier - a magical effect from the ciscircular face disrupting the surface, or a material object sufficient to establish a circuit across the barrier (i.e. about a millimeter across), the resulting anomaly acts as a breach within the pressure wall, with both contained mana and the barrier itself rushing to fill the void, the entire construct collapsing in on itself within a fraction of a second.

Note

Conventional Magic Circles are notably three (plus time) dimensional. This means that aetheric travel to and from a magic circle is possible without disrupting it.  Indeed, this is how summoning works.
While purely magical effects tend to simply break apart upon impact with the transcircular face, ectoplasmic entities have a more peculiar reaction.  Ectoplasm is some ways mana and in some ways matter, and is repelled by the transcircular face, but also has momentum.  The Magic Circle will thus have to apply force to stop it, which it is limited in its capability to do.  Ectoplasmic objects traveling at high speed are thus likely to break through the barrier, though possibly without whatever construct was controlling them, thus causing them to revert to denatured ectoplasm.
Material magics with magic constructs alongside will penetrated and destroy the barrier, but the construct may be damaged in the process.

Casting

Magic Circle is a relatively simple spell, with a dweomer that has been refined, stripped down, and re-refined for the absolute maximum ease of casting, but given its importance methods of reinforcing both construct and effect have been devised.  The most commonplace of these is to mark the affected area with a physical circle - though in practice the caster need only conceptualize the effected area.  At its simplest the caster simply traces out the circle with their fingertips - spinning in place is a common method, but this can quickly become elaborate as methods of increasing the size, intensity, and durability of the effect are sought.  Salt or chalk circles, drawn with the aid of a compass, are commonplace even 'in the field', but at the extreme end elaborate inlaid rings of silver, brass, or even more precious materials are not unknown.
While a magic circle is invisible to mundane sight, it is easly detected by magically sensitive instruments, and a skilled magician can 'feel' a nearby circle through its effect on the flow of mana.

Classification

Tier: One
School
Collegiate: White
Trakkati: Abjuration
Hermetic: Uranus