[since the first two don't sound like anything he's heard of, he leans towards the last option.]

"Something else" most likely. I've never heard of Greek or Roman civilizations. They're the remain of the civilizations that were on the continent of Glenwood before the people there today. Some of them were made with a combination of human skill and seraphic artes, so in at least some cases you could consider them the ruins of the seraphim just as much as humans.