I stayed in the flat for 10 days and enjoyed the stay very much. The location is unbeatable, metro right outside the door and Puerta del Sol is only a 5 minute walk away. The flat was on the 4th floor with a view of the pedestrian zone. The rooms are simply furnished with a nice atmosphere of an old building with high ceilings. The only downside is that it can get a bit noisy from the pedestrian zone. However, it is quiet with the windows closed.
Special thanks to the host who always responded within a short time and took care of my needs.