Its landmarks include the Royal Palace of Madrid ; the Teatro Real (Royal theatre) with its restored 1850 Opera House; the Buen Retiro Park , founded in 1631; the 19th-century National Library building (founded in 1712) containing some of Spain's historical archives; a large number of National museums, and the Golden Triangle of Art , located along the Paseo del Prado and comprising three art museums: Prado Museum , the Museo Nacional Centro de Arte Reina Sofía , a museum of modern art, and the Thyssen-Bornemisza Museum , which completes the shortcomings of the other two museums.