A Shinobi Barrier (忍び結界, Shinobi Kekkai) sometimes called Camouflage Barrier is a battle space created by shinobi to keep out civilians. It has a limited range, and is mostly used for duels. When a shinobi enters combat, it blockades off a space of a fixed size, impenetrable to the average person. It's useful for covert activities, but there is a downside. Since shinobi can sense its activation, it can give away a shinobi's position.
The Shinobi Barrier also serves as a spot where the human world and the yōma world overlap. Those with the proper skill can project their thoughts into this dimension.
Shinobi with enough power can create this special type of barrier. Since those of particularly great will power can enter the barrier, it is commonly used to keep civilians out of a fight, as well as detecting the presence of other shinobi or people wielding power. In the even that one should find oneself trapped within a barrier, eliminating all enemies in the area will dispel it.
A Shinobi Barrier is reminiscent to the original location of the creator.
In the Anime, they have been shown to be able take on a variety of appearances based on the preferences of the creator and thus can be anything they please, similar to a Shinobi Transformation.
Additionally, creating another shinobi barrier within close proximity of another will weaken the pre-existing shinobi barrier.
Linked Shinobi Barrier
This technique was first created by Rin. Since Shinobi Barrier's are created with the mind, If multiple shinobi share the same mindset they can create a larger barrier thus making a linked Shinobi Barrier. Rin achieved this result by altering existing puppetry techniques.
Due to the fact that shinobi barriers possess limited range when created by one person, it is possible to form a linked shinobi barrier to increase it's range. This is done by a group shinobi creating a shinobi barrier at the same time and then merging their barriers together. This technique is not something that is easily done, but there a some shinobi who are capable of this such as a Puppet Master (i.e. Haruka).
Shinobi Blood Barrier
When a shinobi barrier absorbs all the blood it can, it detaches completely from the human world and swells large enough to host a giant yōma.
Shinobi barriers are not impenetrable. If someone from within the barrier acts as a powerful beacon, you can find a way inside. Invaders will face a counterforce emitted by the barrier, and this will greatly tax their bodies. There is also a possibility they may die. Because of this, it is a forbidden technique.
Removal & Exiting
In order to remove a Shinobi Barrier either the creator must take it down themselves or the creator and enemies inside must be defeated, as demonstrated in the games. As long as an enemy stands, the shinobi will not be able to leave the shinobi barrier. Aside from the disengaging a Shinobi Barrier, the creator can also boot people out of a Shinobi Barrier at will.
A poorly made Shinobi Barrier falls at risk of becoming unstable and if the creator is unable to remove the barrier then they will be trapped inside forever. Poorly constructed Shinobi Barriers can be disengaged from the outside.