It says, "Santo Domingo Plaza, Mexico City, 14th Century."

This is the Plaza de Santo Domingo in Mexico City, Mexico. Most of the wikipedia article about this deals with how this is apparently a great place to get falsified documents, so I'd just skip it because I'm betting that's not your thing. There is a good bit about how the fountain there has a statue of Josefa Ortiz de Domínguez, a heroine from the Mexican War of Independence.

Julie, who sent the postcard, says that the Plaza is also a cool place to see concerts, and that the best one she's seen there was a band called "Los Explosivos." I think this is their Myspace page. They look like fun, I think it would be very cool to see them at this venue!