The Seville Cathedral is supposed to be the world's largest; it's immense size is quite a sight. The interior is also impressive with its numerous chapels and tombs. The Spaniards claim to hold Christopher Columbus' remains at the cathedral although the Dominican Republic also claim to hold it. Nevertheless, his tomb at the cathedral is still beautiful with the four kings representing the kingdoms that make up present-day Spain carrying it. The Royal Chapel or Capilla Real housing several royal tombs is off-limits to tourists although I managed to peak through the curtains. Finally, the view from the top of the Giralda Tower aftering a tiring climb of 34 ramps is both breathtaking and rewarding.
The Alcazar, right next to the cathedral, is also worth visiting. We had to catch a train to Granada so I only had about an hour to visit it, but from what I saw, the place shared many similarities with the Alhambra in its architecture and beauty and its garden was gorgeous with its palm trees. I would absolutely suggest a visit.
Both the cathedral and the Alcazar were undergoing restorations at the time of my visit and some restored areas were covered up with life-size pictures showing what was behind it. I've attached a picture to demonstrate the practice.