This is the Acrocorinth, the fortress that stands at the top of old city. Many Greek towns were built at the foot of a hill, with a fortress at the top. The Acropolis of Athens literally means "city rock" or "town hill fortress". Acrocorinth has the same meaning, but for Corinth.
Most were like this one, not like the one in Athens. They were grim military sites, meant to serve as a last refuge during an attack. The one here at Corinth was functioning throughout ancient times and was added to during the Middle Ages. It is the largest and oldest in the Peloponnese. The ruins are an agglomeration of work done by the Franks, the Byzantines, the Venetians, and the Turks, all built atop the ancient foundations.
St. Paul preached at Corinth and it is to them that his letters to the Corinthians is addressed.
At the top of the Acrocorinth, visible in the picture to the right, is a temple to Aphrodite. Greek ruins like this were visited just as eagerly by the medieval pilgrim as they are by the modern tourist.
Princes and engineers long dreamed of cutting a canal across the isthmus here at Corinth. About seven miles of land separates the Aegean from the Ionian Sea. Ships in the Middle Ages had to sail all the way around the Peloponnese, but in modern times, they sail through the Corinth Canal. Here are a couple of photographs that show off this dramatic and heavily-travelled canal, built in the 1890s.
(these photos were found at the Tourist Guide of Greece)