The door leading to the painting of Las Meninas in 1656 is the door that appears in the painting itself.
When Rodolfo Sancho's character travels to the Independence War to save historical hero El Empecinado, he goes by the name Curro Jiménez. This was the name of the character his father in real life, Sancho Gracia, played in the famous TV series Curro Jiménez (1976), which took place in that period. Sancho Gracia had also portrayed El Empecinado in Los desastres de la guerra (1983).
The tattooed men that point the bookstore to Thibaud and Benito are real twin brothers Emilio and José Alcázar, activists and heavy metal fans that can be found in Madrid's Gran Vía most days of the year.
The Spanish collaborator of Thibaud (who later tries to assassinate guerrilla leader El Empecinado) is named Benito. Spanish novelist Benito Pérez Galdós wrote an influential biography of El Empecinado ("Juan Martín el Empecinado") in 1874.
The door leading to 1656 opens to the scene of the painting "Las Meninas" by Diego Velázquez. This artist is later introduced as the one who sketches the profiles of the men from the past who need to be found in the present.