The building that comprises the main area of The Prado, was originally conceived as a Natural History museum.  Real scientific study was just beginning and a great number of Kings, Charles III included, were eager to sponsor scientific projects and buildings.  This gave not only a learned aire to the city, but also to the King's themselves.   It is referred to as the "Villanueva" building after the architect Juan de Villanueva who designed the building. 

Although this building was originally begun in 1785 construction was halted during the War of Independence.  It did not begin again until the reign of Ferdinand VII (1803 - 1833).  During this time  Ferdinand decided that there needed to be a building to display the Royal Art Collection.  This was an idea that had been originally proposed during the reign of Charles III.  It was mostly though the special interest in the project that his wife, María Isabel de Braganza showed that the idea was actually carried out.  As such she is considered to be the founder of the museum.    Rather than place it in an existing building he decided to finish the building which his grandfather (Charles III) had begun 30  years ago.  Even then it was difficult to complete the project because the people of Madrid keep "borrowing" construction materials to rebuild their own homes which had been damaged during the war.  As such it took over 10 years to complete the building which was finished in 1819.  Since then it has been expanded in 1918, in the 1950's and again in the 1960's.  Since that time instead of expanding the present building another surrounding  building
(Casón del Buen Retiro) was incorporated into the museum in 1971. 

The museum almost ceased to exist soon after it was completed.  King Ferndinand VII died and willed half the collection to his daughter, Isabella II and the other half to her sister, Luisa Fernanda.  To avoid the collection being split up it was decided the Queen Isabella II would buy her sister's half.  It was then decided that the collection could only pass from monarch to monarch.  This never became a future issue anyway because in 1868 Queen Isabella II was expelled and the museum was nationalized.

The Prado contains about 9,000 works of arts, mostly paintings.  However, there is only room to exhibit about 1,500 of these.  As such those works of arts that are not part of a permanent collection became part of rotating temporary exhibits.  The list of artists is extensive and include not only Velásquez, but also Goya, El Greco, Brueghel, Van Dyck, Rembrandt, Rubens, Botticelli, Rafael,  Murillo.  The museum collection also includes many 19th century works of art but these are primarily displayed in the Casón del Buen Retiro.

The view above is of the west frontage of the museum.  This is commonly referred to as the "door of Velásquez" because of the statue in front commemorating a century since his death.  Above the door is a frieze.  This frieze depicts an allegory of King Ferdinand VII as protector of science, art and technology which are represented in symbolic figures and are located in front of his royal throne.  Behind the king are the classical mythological gods (Athena, Apollo, Mercury, Neptune). The facade was completed with a double decoration in the two long stretches of the Central Gallery, allegorical feminine sculptures in niches and medallions with the busts of the main Spanish artists, six painters (three on either side of the door), five sculptors (on the left), and five architects ( following on from the ones on the right).  This exterior decoration of the building was actually completed 5 years after the interior of the building.