Following in the footsteps of Park Güell, the Castell de Monjtuïc is yet another stop on your sightseeing tour that you'll have to pay for, though it was free until 3 March. The management of the space has been granted to a private company and, according to the City Council, you'll be slapping down an entrance fee of between €3 and €5.
A reduced rate is available for visitors over 65, the unemployed and large families. You'll also be offered this lower price if you have the Consorcio de Bibliotecas library card (free and available at any library). If you visit the park frequently, you can buy a year-long pass for €8.
The Council justifies the new arrangement by saying that the site now offers a "museum facet" and that the money earned will go towards building an interpretation centre and improving other areas of the castle. Also, they've reopened spaces like the tower, the tank, and the castle dungeons that you can access with a tour guide.