From the World Heritage inscription:
Tárraco (modern-day Tarragona) was a major administrative and mercantile city in Roman Spain and the centre of the Imperial cult for all the Iberian provinces. It was endowed with many fine buildings, and parts of these have been revealed in a series of exceptional excavations. Although most of the remains are fragmentary, many preserved beneath more recent buildings, they present a vivid picture of the grandeur of this Roman provincial capital.
The Roman remains of Tárraco are of exceptional importance in the development of Roman urban planning and design and served as the model for provincial capitals elsewhere in the Roman world. Tárraco provides eloquent testimony to a significant stage in the history of the Mediterranean lands in antiquity.
If you drove through the city of Tarragona you might not think it was anything special, even if you saw the Roman ruins in the middle of town. However, this was one the Roman capital of the entire Iberian peninsula. Julius Caesar, Augustus, Pompey and Hannibal all visited here. As Roman ruins go, Tarraco isn’t the greatest in the world. What is lacks in the spectacular ruins it more than makes up in history. The ruins of Tarraco are an easy one hour drive from Barcelona by car.
View my complete list of UNESCO World Heritage Sites.