For history & culture, Puerta del Sol is hit or miss.
Less a sight than the city's living switchboard: the point every road in Spain measures from, the spot where Madrileños mass on December 31st to eat a grape on each of the twelve bell-strokes, and the everyday meeting place under the bear-and-strawberry-tree statue (the city's heraldic emblem, El Oso y el Madroño). You feel the centrality more than you photograph it.