When the world was nearly destroyed by the continued rampage of the Teras Pharma, 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 Teras Pharma.
In addition, once every 100 years, the barrier becomes unstable, allowing the Teras Pharma to enter the distortion from the outside. The nature of the distortion is the reason that the Teras Pharma are referred to as the centennial monsters as they appear from the inside to only appear once a century.
Only very few people, with unusual magical prowess, would be able to pierce the barrier deliberately. Only Astrid and Zephon are seen to be able to do so. The barrier can be seen from the outside world but from the inside merely looks like continued, unblemished scenery. Approaching its edge from the inside causes a person to seemingly loop the same small amount of space leading up to it, never breaching it.
This effect is what led to the dismantling of all ships in the Holy Empire of Ionia around HIY 100. Ships that neared the edge of the barrier would become hopelessly lost at sea with no reference points to guide them and even the stars proving ineffective for navigation.
Knowledge of the barrier itself would be suppressed around that same time in order that the people of the land not continue living in despair in knowledge of what happened to the outside world. The barrier itself would fall for good with the defeat of Renefarious, allowing unfettered access to the vast open world for all.
- Genso Suikoden Tsumugareshi Hyakunen no Toki The Complete Guide (ISBN 978-4-04-886462-6), page 240