Guillermo's apartment is an excellent choice to discover Madrid, be it walking, by metro or by taxi (a stop is just around the corner, taxis are available at all times). In front of such stop is a Carrefour supermarket, open 24/7. The neighborhood is lively, but one must be careful at night, as you normally would in any big city.
Everything in the apartment is modern, functional and tasteful. There are two separate rooms, each with an actual bed, so 4 people would be perfectly comfortable sharing this apartment. As it's in an attic, there's always a lot of sun light coming in through the windows. The building itself is classic, but one drawback is that there's no elevator. Young tourists might not find difficulty going up & down 4 floors, but in my case I travelled with 2 people with reduced mobility, each with a 23 kilogram suitcase, so it was inconvenient for them to go up the stairs everyday.
Attention is excellent. Antonio, Guillermo's friend, was in charge of giving and receiving the keys, and he was always attentive, caring, available and on time.
I'd definitely recommend Guillermo's apartment, I'd surely stay there again.