We stayed in Maria's small and comfortable flat for 4 nights and really enjoyed it. You have everything you need and the street is rather quiet, so nothing stops you from getting a good night's sleep.
As the title suggests, you can really walk all around the city centre from the flat. Even better, just in the vicinity of the flat you have a lot of nice shops, restaurants and bars.
Maria was very friendly and helpful, so we recommend this flat if you want to stay in Madrid's city centre for a couple of nights.