Seven Walls

The term Seven Walls (七つの壁) indicates an interdimensional 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.