The low hill facing the harbor of Barcelona is a nice place to spend your leisure time. The park hosted the 1929 International Exposition and 1992 Summer Olympics. You can find several sports facilities which were built for the Games here, and also the Olympic Tower designed by Santiago Calatrava.
Don't forget to visit the botanical gardens with beautiful plants and trees or some of the important museums, such as the Fundació Miró and the National Art Museum of Catalonia. The famous Magic Fountain is also set here.
The easiest and fastest way to get to the top is by taking the funicular (included in your metro ticket) and then the cable car, which takes you directly to the castle. If you've got enough time, you can make it a trip and take a walk.
Wikipedia https://en.wikipedia.org/wiki/Montjuïc
Address Avinguda de Miramar, 08038 Barcelona, Spain
Coordinates 41°21'48.68" N 2°9'21.984" E