Two legends say that city is home to gods. so it really does have to be powerful.
Make that three. The Tevinter expected to find the power of the gods.
However, one thing is bothering me right now. There don't seem to be extensive records of the Tevinter attempt to enter the Golden City, or else less people would question it as Chantry propaganda. This rather suggests to me that someone probably covered up the whole incident.
If that was the case, who or what told Andraste about it? She was a nobody born long after the original incident, and in no position to have access to state secrets.
If it was simply common knowledge, then I would think the Dwarves would have known before the discovery of Corypheus. Dwarves keep good records of things and had trade with the Tevinter. You would think they would have written about this kind of thing if it ever became commonly known.
Oh, and lest we forget... The Darkspawn first started appearing underground!
Who would ever make the connection between the Tevinter trying to reach the Golden City and the Darkspawn appearing? Well, I guess someone could remember that the city was Golden and connect it with the Darkspawn that eventually appeared, but if it were that easy than I would think there would be more stories about it than just the Chantry's.
But still, Andraste does seem to have known more than she should have about the whole incident. I suppose that's what I'll say for Andraste...
I think that whatever she may have been, she had an informant who witnessed the invasion of the Golden City.