I have stayed in many Airbnbs in Madrid but this is the best of the lot! I liked it so much I have already booked another stay.
The apartment is situated at the back of the building so completely silent during the day and at night. Being on the ground floor it's very cool -- I did not have to turn the air conditioning on once, even when it was 40C outside. The patio is great for relaxing and also stays cool as it only gets direct sun in the late afternoon. The whole apartment is very nicely designed and furnished, the bed is comfortable and the kitchen well equipped. The wifi is fast and works perfectly if you are working from home.
On top of that, the location is great, being just 5 mins walk from Gran Via and about 10 from Sol, as well as the Tribunal and Noviciado metro stations being about 5-7 mins walk away each. There are two supermarkets close by, one on each of the streets running parallel to the apartment, with plenty of bars, cafes and restaurants close at hand. You are really in the heart of the city at Marta's place.