Carlos' place is simply great!
It is located very close to Sol square, which is very much the heart of Madrid. We walked to all the places we wanted to visit (the Palace, Prado museum, Retiro park, Riviera music club, etc.), except the Zoo. There are quite a lot of bars and restaurants working late nearby and there are people on the streets late in the evening.
The place itself is small, but very comfortable for a couple. It has a big bed, a small table for eating and a very well stocked kitchen (we used the stoves, the dishwasher and even the washing machine). Everything was sparkling clean. There is a TV in the small living room with Netflix, which was very handy during the few rainy days we encountered.
Carlos met us personally for the check-in and showed us around the apartment. His English is good enough to have no problems in the communication with him (especially in written form). He had even left food and drinks for us in the kitchen, which was extremely nice of him!
I wholeheartedly recommend Carlos and his apartment in the heart of Madrid! If you visit the city and look for a place for two people, look no further!