The cathedral situation in Zaragoza is quite confusing and relatively complex.
For starters, the city has a co-cathedral arrangement. Both the Cathedral-Basilica Nuestra Señora del Pilar (seen in this posts cover image) and the Cathedral del Salvador (more commonly known as La Seo de Zaragoza) are just a few hundred feet apart in the city square.
La Seo de Zaragoza looks like the lesser of the two cathedrals because from the outside it is considerably smaller and appears less beautiful. Walking inside, paying the entrance fee, and getting an audio guide (offered in several languages) that explains each individual chapel tells you a completely different story. It is the most beautiful and serene cathedral I have ever seen in my life. I mean that in all seriousness and having seen my fair share of cathedrals in the 22 countries I've visited.
**Pictures are NOT permitted inside La Seo cathedral**
The Cathedral-Basilica Nuestra Señora del Pilar will certainly be the first one to grab your attention as you mull around Zaragoza with it noticeable size and four towers. As a matter of fact it's the second largest cathedral in all of Spain (behind the cathedral in Seville). If you see it at night it is beautifully lit up along the river. Don't be fooled though. It is not as old, and, while still pretty in its own right, especially from the outside, it doesn't quite compare to it's neighbor.
Make sure to take a stroll around the Cathedral-Basilica and look up at the outside walls. You'll be sure to notice the many holes in the walls. Those are the what remains of cannon ball fire from Napoleon's take over of Spain in the Peninsula War.
When visiting Zaragoza, you ultimately want to visit both cathedrals as they both are an integral part of Zaragoza's history and aesthetic.
Aside from the cathedrals, Zaragoza is noticably famous for the Aljafería Palace. This palace was built in the late 11th century during the reign of the Banu Hud dynasty. It is the only conserved large example of Spanish Islamic architecture from the era of the taifas (independent Islamic Kingdoms). Today it houses the government of the autonomous community of Aragon.
The parliament can also be toured when touring the Aljafería Palace.
Other interesting things to check out while in Zaragoza are the original roman wall and the statue of Ceasar Augustus that was donated to Zaragoza by Italian dictator Benito Mussolini. A walk across the Ebro River on the Puente de Piedra bridge and stopping for a few moments to enjoy the views is also worth your time. May I suggest not wearing a hat as the wind on the bridge can get pretty intense.
Zaragoza is known as being Spain's windiest city. Zaragoza is situated pretty low above sea level in a valley and the wind apparently swirls down the Ebro River, the second longest river in Spain.