You've correctly identified that we're looking at comparisons here, and we want to compare this other civilization to the city that Homer's heroes knew. Whenever we have a comparison, we have to ask - are we comparing comparable things?

(A) Here we're comparing "civilization" to "city," which is what we want, but clearly the "as" makes no sense here. We need "than."

(B) Here we have "than," and we're comparing "civilizations" to "city," so keep this in the running.

(C) We have a "than," which is good, but what are we comparing? When we introduce the word "was," it changes what we're comparing! We could say "the civilization was older than was the city known..." Then we're not just comparing nouns, we're comparing the clauses. But since "civilizations" doesn't have a verb, we can't add a verb for "cities."

(D) "more ancient of a city" is a colloquialism that is idiomatically incorrect.

(E) Ditto "older of a city"

So, you're probably right that "older than" is more concise that "more ancient than." But remember - concision is a preference, not a grammatical rule. Make sure you deal with all the grammar rules first, and save concision decisions for last. ("Concision Decisions" would be an awesome band name, right?). Grammatically, we can eliminate everything but B.

Hope this helps!