Carlos' flat is very conveniently located about 5-10 minutes walk from Retiro park and the Prado Museum. Calle de Lagasca also appears to be a good place for shopping with a number of designer boutiques. The apartment has everything you need for a short stay in Madrid. Although I was fine the apartment block is relatively noisy if you're a light sleeper.
Although I never met him, Carlos was always helpful and responsive via email and organized an early check in.