This massive 1884 building, Spain's central bank, takes up an entire block. It's said that part of the nation's gold reserves are held in vaults that stretch under the Plaza de la Cibeles traffic circle all the way to the fountain. (Some reserves are also stored in Fort Knox, in the United States.) The bank is not open to visitors, but the architecture is worth viewing.
Paseo del Prado s/n, at Pl. de la Cibeles, Madrid, 28014, Spain