La Sagrada Familia is one of Gaudí's most famous works in Barcelona. It's a giant Basilica that has been under construction since 1882 and it is not expected to be completed for another 30 to 80 years.
Antonio Gaudí played an active role in directing the construction of the Sagrada Familia until his death in 1926. His architectural designs and schematics were destroyed by accident and this is the second most aggravating factor in the length of time needed to construct the building. The primary being money. Ownership of the project has changed several hands due to the sheer amount of money needed to fund the construction.
Interpretation of the original designs by present day architects is particularly challenging because computers compare their work to what "it" believes was Gaudi's intent before it is added to the building.
The building is still under construction so be prepared to see a lot of work continuing when you visit. However this in itself is interesting, especially if you visit the museum inside the building.
I also recommend doing the audio guide tour as me and my friend Mireia did in the picture.
Click on the picture below to view a large panaramic version I made by stitching photos I made together. It really is an impressive place.