A grand cathedral on the highest point of the old town, begun in 1171 in Romanesque style and finished in Gothic in the 14th century — the transition is visible in the change from rounded to pointed arches as you move from the eastern end to the western. The cathedral sits directly on the foundations of the Roman Temple of Augustus, the most important religious building of Roman Hispania; the medieval builders reused Roman columns and capitals throughout the structure.
The cloister is the highlight: 12th-century Romanesque double columns, hundreds of finely carved capitals (look for the famous "rat funeral procession" capital, where a procession of mice is shown burying a cat — a satirical comment on the secular clergy), and a quiet garden of orange trees. Combine with the small Diocesan Museum next to the cloister, which has a fine collection of medieval reliquaries.