"Helen" is about the Greek opinion of Helen of Troy, ex-Greek, current Trojan, who lives in Troy… or at least she did in the wayback days. So where does this poem take place? Not really anywhere we can point to on a map. That's because this one's about thoughts and feelings. It's not about action. We don't hear about the fantastic battles, the Trojan horse—any of that violent stuff, and we don't hear about the scenery, which is kind of a bummer, since we hear Ancient Greece is beautiful this time of year.
When we read "Helen," we have to remember that this vision of Helen is all in the Grecian people's minds. We read about a few traditionally Greek items—olives, cypresses—but on the whole the poem is interested in describing Helen, not Greece itself.
Interestingly, the poem is written in the present tense. The first line of the poem is "All Greece hates"—and not, for example, "All Greece hated." So the poem feels very ongoing to us, even though Helen is a figure from the very distant past. The effect of this is that the poem feels so incredibly relevant; Helen is as important a figure today for Grecians (and the world) as she was thousands of years ago. That tells us about the sheer power of myth. Helen, who may not have ever existed, seems just as alive today as she did thousands of years ago, in legends.