Bounded Field

A Bounded Field (結界, Kekkai) is a topographic type of ninjutsu that consists in knitting a network of chakra 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. The most powerful type of Bounded Field makes the space itself act as a boundary line and it is on an S-rank level.

Mechanics
Once constructed, the established boundary line will delimit the range of the powers of the Bounded Field, but it is possible to engrave a stream of seals across a the interior of the field to expand the area of effect and strengthen it. Removal of the seal by a third party is possible as long as the technique used to engrave it isn’t too advanced. In this case, it is only possible to temporarily hinder the Bounded Field by dissipating the chakra used to activate it, but the creator of the Bounded Field can amend this by simply putting more chakra 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 with 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 chakra interference their efficiency against those who can protect themselves via barriers 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 sensor. An ideal Bounded Field is the 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 sensor.