Our daughter was working in Madrid for three months and found this charming little posada right in the heart of MAdrid. The Cava Baja is full of character. Apparently the King of Spain eats in the restaurant a few doors away nearly every week.
The Posada was a converted winery and they had cleverly created glass floors so you could see down into the cellars. WE stayed up the top in a wonderful room with lods of character, with a bath with a jacuzzi [as well as a shower].
Charming bar with a great choice of wines. The breakfast was good.