I would say "At a square like this?" or "In a square like this?" "On" a square would be my last choice. More correct answers must be included in this case. In Czech one can say: "Jsem na náměstí" but in English, the best translation would be "I am in/at a square." Not "on" it.
Interesting. Google claims the occurrence to be as follows:
Nevertheless, AT should be acceptable as well.
I (native US English speaker) also would say "ON a square like this." I might use "at" if I wanted to say, for example, "I'll meet you at THE square." I probably wouldn't use IN "with either a" or "the" square.