On our last day in Barcelona, we walked to La Sagrada Familia (Holy Family). This church (it is not a cathedral) has been under construction for over 100 years. Originally designed by Antonin Gaudi, it has been added onto and expanded by other architects after his death, funded by admission of more than 1.5 million guests per year.
Admission was 8€ and worth every penny. The entrance is the Nativity Facade, and if you look waaay up high, mosaics spell out "Sanctus, Sanctus, Sanctus." You will get a better view of these if you climb the inside towers. There is a great deal of symbolism in these facades, such as numbers adding up to 33, the number of portal sections (3 - the holy trinity), towers (12 - apostles), etc. I highly recommend bring a good tour book (we swear by Rick Steves) or renting the 3€ audioguide.
We then headed to the stairs for the tower. We could have taken a lift for 2€, but where is the fun in that? Instead, we spent the better part of an hour slooowly climbing the spiral staircase and joking around. To reward us for our long wait, we had fantastic views of the spires and construction, as well as the city of Barcelona.
From the interior of the church, you get a first hand look at the massive ongoing construction. Stained-glass windows are still being put in, as were columns and floors. The interior is as beautiful as the exterior, with a canopy of trees standing in for structural pillars.
To say that you must see this is an understatement. Over one million people stream through here each year because it is the most fascinating site in Barcelona.