A Bounded Field (結界, Kekkai?, lit. "Barrier") is a topographic type of Magecraft that consists in knitting a network of magical energy and spreading it on base area, such as a piece of land or a building, to create a mystic boundary line that separates the inside from the outside. By definition, it should not be possible to move them once they are set, but those created by Souren Araya and Sancraid Phahn are an exception to this rule. The most powerful type of Bounded Field makes the space itself act as a boundary line and it is on a level close to Magic.
Once constructed, the established boundary line will delimit the range of the powers of the Bounded Field, but it is possible to engrave magic sigils across the interior of the field to expand the area of effect and strengthen it. Removal of the sigils by a third party is possible as long as the technique used to engrave them isn’t too advanced. In this case, it is only possible to temporarily hinder the Bounded Field by dissipating the magical energy used to activate it, but the creator of the Bounded Field can amend this by simply putting more magical energy back into it. In the Fate/stay night anime this was changed to the sigils being easily destroyed. However, new sigils are constructed by the actions of the old ones, making the Bounded Field capable of self-repair in due time.
As their original purpose was for the creation of safe havens, the effects that can be set within a Bounded Field are in their majority those whose aim is to protect the caster. Offensive variants do exist, but as they work through indirect magical interference, their efficiency against those who can protect themselves magically is low.
Advanced Bounded Fields work on a subconscious level to prevent onlookers from noticing it, but placing those on a large area increase the chances of being detected by a trained magus. An ideal Bounded Field is one that cannot be detected by anybody. On the other hand, a Bounded Field that can be easily noticed is seen as the mark of a third-rate magus.
Medea's advanced skill in magecraft allows her to construct an "another world", that is on the same scale as a Reality Marble, though the time to prepare such a thing is extreme. It is shown in the Fate/stay night [Unlimited Blade Works] anime as a simple re-creation of the Fuyuki Bridge, with a foggy interior. She used the lingering traces of the previous war's Caster's summoned Horrors to create it. Medea also shows that with more time and effort something similar to "another world" can be constructed, which she refers to as being a sort of Reality Marble, where she has prepared her own city. Caster even claims the Doll-people inside are "literally" alive, because she takes the time to make each and every one by hand. It is said that Medea displaying the ability to build elaborate miniature worlds, the "most basic and most advanced principle of magecraft," is proof that she learned directly from Hecate.
Examples of Bounded Fields
- Touko Aozaki's Bounded Field
- Emiya residence barrier
- Dead Apostle Territorial Fields
- Blood Fort Andromeda
- Breaker Gorgon
- Hounouden Rokujyuyonshou
- Invisible Air
- Rho Aias
- Rokudou Kyoukai
- Ryuudou Temple’s Barrier
- Kayneth El-Melloi Archibald’s twenty-four-layer Bounded Field protecting his hotel floor
- Sancraid Phahn's personal Bounded Field that moves with him
- The Mist
- Crystal Palace
- Ragnarök Lífþrasir
- Akhilleus Kosmos
- Ganesha Vighneshvara
- Doomsday Come
- The "Garden"
- Medea's "Other World"
- Vampire-containing Bounded Fields used by the Holy Church
- Fate/stay night - Prologue: Day 3
- Fate/stay night [Unlimited Bladeworks] - Episode 12: "The Final Decision"
- Kinoko Nasu Twitter 12:17 PM JST · Dec 27, 2014 - "The mirror world is a polluted world. The bridge has always been a boundary, a sign that separates this world from the other world. In addition, this is a river that was contaminated ten years ago when the evil gods of the other world were summoned. Taking advantage of this, Caster has shifted the entire phase and invited her target in."
- Fate/Grand Order - Medea's Interlude: Witch's Backyard
- Fate/Grand Order material II - Medea, translated by ComunCoutinho on Reddit
 Fate/strange Fake - Volume 6 - Servant status: Rider[T]
Master: Kuruoka Tsubaki
True Name: Pale Rider
Sex: The concept does not apply
Height and Weight: Varies based on infection and diffusion (roughly equivalent to a parvovirus at minimum)
Alignment: True Neutral
Noble Phantasm: EX
■ Anti-Magic: C
■ Riding: EX
■ Infection: A
The ability to expand its territory by infecting other organisms with parts of itself in the form of bacteria or viruses. The minds and bodies of the infected are under Pale Rider’s control, and their minds are drawn into the world created by its Noble Phantasm. Their magical energy may sometimes be absorbed as well.
■ Innocent World: EX
A skill that reflects conceptions produced by human fears of “death” and “pestilence.” Because these conceptions are so varied, Pale Rider is a nondescript entity when it is first summoned and the tendencies of its very being change depending on those it draws into the “underworld” of its Noble Phantasm.
■ Guidance of the Underworld: EX
The ability to grant a variety of blessings to allies that Pale Rider has drawn into the territory that has become an underworld due to its Noble Phantasm. Because Pale Rider is not itself the ruler of the underworld, this skill differs slightly from a certain deity’s “Protection of the Underworld” skill.
■ Doomsday Come: Come, Dark Path, Come
Type: Anti-World Noble Phantasm
Maximum Targets: —
A Noble Phantasm that creates a Bounded World that becomes an artificial “underworld” with Pale Rider’s Master as its starting point, which acts as a receptacle for the “deaths” that Pale Rider bestows. Because it is strongly influenced by the Master’s conceptions, it can become an archetypal heaven or hell, but it can also become a space that acts as absolute nothingness and destroys souls. In extreme circumstances, Pale Rider can pull its target’s body into the ward as well as their mind. The ward is typically smaller in scale, but as a result of connecting to the land itself and other factors, Pale Rider is currently creating a larger ward than it would when summoned normally.
■ Kagome Kagome: Sword, Famine, Death, Beast
Type: Anti-Army Noble Phantasm
Maximum Targets: 999
The ability to manifest and utilize the power of numerous objects that impart “death” to others within Pale Rider’s ward. In an ideal environment, it is also capable of recreating a mythical “doomsday” to the extent that magical energy will permit. It did not reach that level because Tsubaki lacks knowledge of the Apocalypse or Ragnarök and did not wish for a hell. The pronunciation of this Noble Phantasm’s name varies from Master to Master.