If I put myself in the position of the leadership...
If I knew and wanted to properly protect and hide the most important thing for the entire city-state...
How would I do it?
Considering the current situation I'm already aware of...
To retrieve this Treasure, the approval of the most powerful people, or those with the highest authority among these four city-states, would surely be needed.
No one would want others to take this thing freely, nor would they want others to take it without their consent.
After all, it concerns the survival of the Race.
Considering the Difficulty of that Treasure, 'Four Parts of Power'... it becomes very clear.
It would require the presence of the four Priests with the highest authority to unlock and obtain this Treasure.
What could represent them? Undoubtedly, it would be their most powerful Holy Grail Level Treasures.
The information also clearly states that each one is controlled by a Priest.
