I had a wonderful stay at Dulce's place. It was a small apartment located in the "older" part of the city. It was a walking distance to most landmarks, including "Zocalo" (the central plaza). There were hundreds and hundreds of shops in the neighborhood.
An interesting thing about the unit was that it did not share any walls with the neighbors and it had generally very low ambient noise, which I liked. One thing to note is that because it was located on the second floor (inside a 5~6 story building), it did not have much natural light, if that matters to you.