Guillermo's apartment was one of my all time favorite places to stay! It is beautiful, comfortable, and has everything you need. The neighborhood is awesome and accessible, with plenty of great restaurants (I'd recommend Federal Café, Crumb, and Roll) within a two minute walk and some great shops (Mini, Sportivo, How) in the same square as the apartment building. The bed is comfortable, the bathroom is spacious and clean, the kitchen is equip with everything you need, and there's central air conditioning to keep the temperature just right.
When I arrived I was met by an awesome and friendly guy named Antonio who showed me around the apartment and told me everything I needed to know. He sent me a few messages during my stay to check in with me and make sure I was comfortable and enjoying the apartment.
I couldn't recommend this apartment anymore for your stay in Madrid!