Marcos' place is perfectly located in Madrid's center.
We were able to walk everywhere and only needed to ride the train to visit Toledo and Alcala de Henares.
We were lucky too, that Marcos' appartment was two blocks away from where the celebrations of Virgen de la Paloma take place. If you are visiting around August 15th, you definitely should take part.
The place was also very clean and has everything you need to have a perfect stay in Madrid.
Marcos was also very responsive and the logistics (check in, etc...) went perfectly.