Seven Walls

The term Seven Walls (七つの壁) indicates an inter-dimensional place, accessible only through a particular "entrance", which represents an imaginary barrier that separates from the rest of living beings. The Seven Walls are actually an allegory to indicate space and time, and therefore the only way to go beyond the Seven Walls is to transcend the laws of space-time, using one's own subconsciousness and imagination within this place.

History
Not much is known about the history of the Seven Walls, but over the centuries, various legends have spread about this place, which tells that in the Seven Walls, it is possible to meet the highest-standing demon, the Demon King. The place, however, is not marked by any map, and nobody has succeeded in finding its exact location for centuries. The only clue is represented by an apparently-indecipherable enigma, which has spread with the passing of generations, up to James Ratri.

Promised Forest Arc
Mujika gives Emma an amulet as a good luck charm and tells her about the Seven Walls. She later notes that there is more than one Promise.

Search For Minerva Arc
James Ratri, unable to find the solution to the puzzle, decided to include the information inside the memory chip he left at Goldy Pond, hoping that in the future someone could recover it.

Seven Walls Arc
Seeking to find a better solution than Norman's genocide, Emma and Ray venture to the Seven Walls. They enter and become quickly confused upon entering. as the Day and Night does not follow the laws of physics as they are used to. They are first brought to an eerie rendition of Grace Field House spawned by their subconscious, and as they venture they become more and more lost as different routes lead to the same place and the same routes lead to different places, causing them to end up in the same room over 100 times, as counted by Ray. Throughout, they hear voices saying "let's play", presumably due to Him's desire to play with them. Due to the unstable time of the Seven Walls, Emma and Ray increase and decrease in age numerous times. They typically become small children, but one point, Ray ages forward so much he forgets who he is, but is saved from getting lost in space and time by Emma, who has finally figured out the nature of the Seven Walls. After reversing time as Emma instructed, Ray goes back to the normal Demon World and Emma goes to the Day and Night to make the new promise with Him.

Trivia

 * Yverk and Julius used a trick to enter the Seven Walls. Almost no one else from the Demon World knows about these tricks.