The Age of Enlightenment lives on in these formal gardens next to the Museo del Prado and at the southwest corner of the Parque del Retiro. Carlos III, the so-called mayor-king, had Juan de Villanueva (architect of the Prado and the Paseos) design the gardens as a collection of temperate zone plants from around the world. The king himself opened the gardens in 1781. Today the meticulously maintained gardens contain more than 104 species of trees and 3,000 types of plants. Nine self-guided tours focus your interests.