Well this is interesting...

[Enkidoh]

Returning to the entrance to the Wanderer's Palace having Oschon's glyph, another member of the Twelve whose symbol is found on ancient ruins is Azeyma the Warden, the goddess of the sun. If you look closely various ruins scattered around Thanalan (such as those at the Silent King in Western Thanalan, the impressive statue in the gorge at Highbridge in Eastern Thanalan, and the Invisible City, also in Eastern Thanalan), you can see they clearly bear Azeyma's glyph, which makes me wonder exactly who it was who built these ruins.

I did think maybe they were Belah'dian, but given the fact that the Sunken Temple of Qarn was mistakenly believed to have been built by Belah'dia, but in fact predated it, I'm wondering if there was an even earlier civilization that existed in Thanalan, long before the lalafell arrived, one that worshiped Azeyma, much like the Seekers of the Sun miqo'te years later. It's an interesting thing to think about in any event.

[Alenore]

I did think maybe they were Belah'dian, but given the fact that the Sunken Temple of Qarn was mistakenly believed to have been built by Belah'dia, but in fact predated it, I'm wondering if there was an even earlier civilization that existed in Thanalan, long before the lalafell arrived, one that worshiped Azeyma, much like the Seekers of the Sun miqo'te years later. It's an interesting thing to think about in any event.

Well the civilization that used Black Magic during the War of the Magi probably was from Thanalan. After all, Belah'dia is in Thanalan, and Ul'dah/Sil'dih are now in Thanalan as well.
However, the symbol of Azeyma could have been added by Belah'dians in Qarn. I doubt they just took the temple and left it how it was.