The subject of this article has not yet been localised outside of Japan by Konami. Please remember that these romanisations are non-canon.
When the world was nearly destroyed by the continued rampage of the Terrus Firma, the sages made full use of their sorcery to create a barrier that the monster could not penetrate. Although the small amount of land covered by the Barrier was protected, the rest of the world was plunged into catastrophe by the Terrus Firma.
In addition, once every 100 years, the Barrier becomes unstable, allowing the Terrus Firma to enter the distortion from the outside. The nature of the distortion is the reason that the Terrus Firma is referred to as the Centennial Monster as it appears from the inside to only exist once a century.
Only very few people, with unusual magical prowess, would be able to pierce the Barrier deliberately. The Barrier can also be seen from the outside world.