To begin building the Great Oracle was the last order the Thirteen gave before they left Terenus to guard it from afar.
Despite the tireless efforts of thousands of workers, it's construction only finished around
300 NV.
Since then, it has been the center of the church of the Thirteen.
These workers were also the last commoners to see the Oracle in all its glory.
Only days later—as a consequence of the
First Prophecy—the ban was instantiated.
As such, all accounts of the Oracle's beauty were quotes of their tales.
Within a few generations, this oral tradition led to the habit of finishing such stories by saying "A least that's what they say, I've never been there."
I really like the visuals in the architecture section, they're vivid and give a very good idea of the purpose and nature of the great oracle