Short answers, pulled from the story.
King Philip II of Spain laid the cornerstone for El Escorial in 1563. This event marked the beginning of a project commissioned to commemorate the Spanish victory at the Battle of St. Quentin on the 10th of August 1557 against King Henry II of France.
Juan de Herrera created the architectural style known as the Herrerian style after taking over from Juan Bautista de Toledo following his death. The gridiron floor plan emerged fully when Herrera eliminated six interior towers from the original conception.
The Royal Pantheon crypt serves as the burial place for kings of Spain since Charles I and contains twenty-six sepulchres holding remains of monarchs from Habsburg and Bourbon dynasties. The most recent monarch interred there is King Alfonso XIII removed from Rome in 1980 while Victoria Eugenie of Battenberg was interred in 2011.
Currently the library holds over 40,000 volumes including many important illuminated manuscripts such as the Ottonian Golden Gospels of Henry III dated between 1045 and 1046. A fire in 1671 destroyed about 5,280 handwritten codices but saved printed collections which now include more than 600 incunabula and thousands of 16th-century books.
On the 2nd of November 1984 UNESCO declared The Royal Seat of San Lorenzo of El Escorial a World Heritage Site marking its global significance. It is now a popular tourist attraction often visited by day-trippers from Madrid with more than 500,000 visitors coming every year.