One of few vestiges of the 17th-century Palacio del Buen Retiro, this austere building overlooking the Parque del Buen Retiro has been restored and now hosts an academic library by the Museo del Prado. There are occasional guided visits to the stunning Hall of the Ambassadors, crowned by Luca Giordano's astonishing 1697 ceiling fresco The Apotheosis of the Spanish Monarchy.
Visits can be arranged at the ‘Educación’ desk just inside the Puerta de los Jerónimos in the Museo del Prado. The remainder of the Casón del Buen Retiro has been renovated in a modern style.