When in Barcelona, make sure to stop by La Sagrada Familia, a Roman Catholic church designed by Catalan Spanish architect Antoni Gaudi. While seeing the structure from the outside is rewarding enough for many, the few euros required to enter are well worth it. Attracting more than three million people per year, the conglomeration of tall towers has been under construction since it was erected over 100 years ago. It's a work in progress - each year leaves it with stronger restorations - but it's still a spectacular attraction that cannot be missed.