I recommend this apartment. It is smaller than pictures show, and the single beds are not very nice. However, it's cozy, well-located and the host is very helpful. It was difficult to find my way around because the little street does not appear in maps. But it did not take too long to figure out. It is close to San Miguel Market, Puerta del Sol, the main piazza etc.
It is important to mention that the place is not very luggage friendly. Prepare yourself to carry your luggage up and down stairs to access the street and then some more in the building before you can take an elevator. But it was definitely worth it for us! Thanks Susana for helping us with local tips and for arranging a taxi. We had a very nice time in Madrid.