Great shots! Where I went to University there is a Greek-style amphitheater (called, curiously enough, the Greek Theater) built into a hillside. They often hold concerts there. It was also the location of my class' graduation ceremony. Not nearly as big or as old as this one, but your photos stirred my memory. :)
After a second glance, this theater may be about the same size as the one I'm talking about. But I'm sure it's much, much older. ;)
The atmosphere at this amphitheatre is terrific. I would love to have attended a performance there but there were none on the three days we spent in the town. The previous performance there was the opera Tosca. I drooled at the thought.