Carlos' flat is first extremely well located to visit Madrid. The balcony over Plaza Mayor is just great.
Secondly, we spent 5 days in the flat with 2 families of 5. The flat is just perfect for 10.
The flat is Well furnished for a nice stay.
The downside is that the neighborhood is living so noisy. It can disturb when sleeping.
We highly recommend the Carlos's flat for discovering Madrid.