I went to Madrid for a few days with a friend, and Marina's place was perfectly located in the Trafalgar area - seconds away from the metro and several great bakeries! Only a 10-15 minute stroll from Sol (a nice walk too). Marina kindly provided us with all the information we could require, and was very clued up on the history side of things.
The apartment itself is modern and contained with everything you could require including a rainfall type shower and a convenient foldaway bed. There's also a bialetti coffee maker and everything you could want in terms of utensils / kitchen accessories.