Diego's apartment is in a perfect location in Madrid's La Latina barrio. It is about a block or two from the La Latina metro stop (closed until September 3, 2017) and within easy walking distance of Plaza Mayor, Puerta del Sol, the Prado and many other major tourist attractions.
The apartment has been recently remodeled in a modern Scandinavian style and has everything you'll need for a long or short stay. The only minor negative is that the windows open onto the building's ventilation shaft and not onto the street, but this is clearly reflected in the budget nightly price.
I highly recommend this listing and hope to stay in the apartment again next time I am in Madrid.