The Iglesia de Nuestra Señora de las Mercedes (Church of Our Lady of Mercy, Las Mercedes at José Reyes, free) is one of the most historic in all of Santo Domingo [1] and one of the most frustrating to get in to see since its hours are notoriously irregular.
It was built in the 1530s and was, of course, sacked by Sir Francis Drake. It was reconstructed afterward but it suffered hurricanes and earthquakes as well. Perhaps the most alluring aspect of the church (aside from the fact that it’s still standing) is its elaborately carved mahogany altar and pulpit with a demon serpent.
Note the cloister adjacent to the church; it is in its original condition. A Spanish monk and writer, Tirso de Molina, lived here from 1616 to 1618; he was in his late 20s or early 30s at this point in his life and was beginning to hone his skills with works for the stage in a form called comedia nueva.
Links:
[1] http://www.moon.com/destinations/dominican-republic/santo-domingo