La Seo Cathedral is, in my humble opinion, the most stunning cathedral in Zaragoza, even more than the world-famous Basilica de Pilar. It's located at the east end of the Plaza de PIlar, right behind the Roman Forum museum.
Externally, it's geometrically-stunning Mudéjar wall of the Parroquieta gives you an indication of the granduer which lies inside. The whole cathedral is done in white ceilings and columns and there are some of the most ornate and complex chapels I've ever seen. Back in the day, only the wealthy could afford to add a chapel or mausoleum to the walls of the cathedral and it became, I imagine, something of a status symbol. You can see that each family tried to out do the others in terms of beauty, ostentatiousness, and opulence.