The top of the hill of Montjuic was the site of several fortifications, the latest of which (the Castell de Montjuïc) remains today. The fortress largely dates from the 17th century, with 18th century additions. In 1842 the garrison (loyal to the Madrid government) shelled parts of the city following disturbances. It served as a prison, often holding political prisoners, until the time of General Franco. The castle was the site of numerous executions. In 1897 an incident popularly known as Els processos de Montjuïc prompted the execution of anarchist supporters, which in turn lead to a severe repression of the workers'' struggle for their rights. On different occasions during the Spanish Civil War, both Nationalists and Republicans were executed there, each at the time when the site was held by their opponents. The Catalan nationalist leader Lluís Companys i Jover was also executed there in 1940, having been extradited to the Franco government by the Nazis.
In the book Shadow of the Wind, the author Zafon refers to this place as the prison where the father of Clara Barcelo (hi was locked up. He would never come out.
Source : WIkipedia