Cristina's apartment is very conveniently located next to the Atocha Renfe metro station. However the trade off is that there is a lot of noise from the street, even with the double glass doors (drivers in Madrid love to use their horn and there are a lot of buses passing by). It is something to take in mind if you are not used to heavy traffic passing close by your bedroom.
The apartment is very nicely furnished and was sparkling clean when we got there. Cristina & Zarai always answered our questions within a few minutes (seconds even). Thumbs up for their reactivity :-)
We had a pleasant stay in Cristina's place, and would surely recommend it to other travelers.