We stayed for 5 nights in Carlos's apartment, which is spacious, clean and (important for us) quiet, with no noise to disturb your sleep. The lift to the seventh floor was particularly welcome after a long day visiting the sights. The kitchen is well equipped when eating in, with ample fridge and freezer space if required. We were there in November so did not use the outside terrace but could see what a great feature this would be for Summer months!
Whilst the apartment is not centrally located, Plaza Mayor is a 15 minute walk, and Atocha station about 30 minutes. However, buses to both these locations leave from the bus stop just outside the front door, and single tickets can be purchased on the bus if you don't need to buy a book of 10. Many of the 'must do' sights can be reached via the 41 bus, eg Reina Sofia, Prado, Parque del Retiro and others.
The WiFi worked fine and a phone/MP3 player can be plugged in to the PC speakers in the office, and music on a USB stick can be played on the smart TV and also on the mini hi-fi system in the main bedroom. Carlos has an extensive library of CDs and travel books for visitors' interest. There was plenty of visitor literature available too, to help plan your time in Madrid.
We'd recommend visiting a lesser known gem, Museo Cerralbo, which is a delightful little palace, quite close to Palacio Real (which is a 'must do').
We ate out mostly in Pl de Santa Ana and Calle de la Cava Baja in La Latina, where there are plenty of tapas bars and table service restaurants from which to choose.
Thanks Carlos for making our stay in Madrid so enjoyable.