Ciutadella Park in Barcelona is open daily, roughly 10am to 9pm or 10:30pm in summer and 10am to 7pm from November through March, with free admission year-round. It sits in the Ciutat Vella district near El Born, reachable via the Arc de Triomf or Ciutadella-Vila Olímpica metro stations.
The park’s centerpiece is the Cascada Monumental, an elaborate fountain that a young Antoni Gaudí helped design as an architecture student, along with the Castell dels Tres Dragons building, a small lake with rowboat rentals, the Parliament of Catalonia, and the Barcelona Zoo.
Most visitors spend 1-2 hours strolling the grounds and fountain, longer if renting a rowboat or visiting the zoo.