The Parc de La Ciutadella Barcelona
La Ciutadella (which means citadel), was constructed as a fortress to house 8,000 soldiers by Felipe V in 1714. Eventually the land was ceded back to the city for use as a public park in 1869. It was officially opened for that purpose for the 1888 World Exposition. A number of buildings still remain from those early years, though they have now been put to use in the Zoo and Geology Museum that are on the grounds.
Of special interest is the Cascada. This beautiful waterfall/fountain was constructed to hide some necessary plumbing, but has now become the centerpiece of the park. Gaudi assisted in the project as a young architectural student. The rock, cascading water and plantings create a piece of art that seems completely natural in this setting.
This is a genuinely tranquil place. It makes for a perfect respite from sight-seeing and fending off street vendors. You may find it hard to leave once you get here.