From the World Heritage inscription:
Cordoba’s period of greatest glory began in the 8th century after the Moorish conquest, when some 300 mosques and innumerable palaces and public buildings were built to rival the splendours of Constantinople, Damascus and Baghdad. In the 13th century, under Ferdinand III, the Saint, Cordoba’s Great Mosque was turned into a cathedral and new defensive structures, particularly the Alcázar de los Reyes Cristianos and the Torre Fortaleza de la Calahorra, were erected.
The Historic Centre of Cordoba now comprises the streets surrounding the monument and all the parcels of land opening on to these, together with all the blocks of houses around the mosque-cathedral. To the south this area extends to the further bank of the River GuadaIquivir (to include the Roman bridge and the Calahorra), to the east to the Calle San Fernando, to the north to the boundary of the commercial centre, and to the west to incorporate the AIcázar des los Reyes Cristianos and the San Basilio quarter. The city, by virtue of its extent and plan, its historical significance as a living expression of the different cultures that have existed there, and its relationship with the river, is a historical ensemble of extraordinary value.
Of all the UNESCO sites I have visited in Andalusia, I enjoyed my short time in Cordoba most of all. I found the old town to be quaint, peaceful and relaxing. The center of the city is the mosque/cathedral. As far as I know, it is unique in the world as it is both a former mosque and the current cathedral for Cordova. After the reconquista, the Catholic Church took over the central mosque and built a christian church right in the middle of the building. It makes for a fantastic architectural clash.
Cordova is easy to get to by high speed train from Seville or Madrid and is definitely worth the trip. It is one of the places in Spain I would love to return to, to stay for an extended period of time.
View my complete list of UNESCO World Heritage sites.