View looking north at the Isthmus that connects the mainland of Greece (to the right and top of image) with the Peloponnese (lower left quadrant). The isthmus was about 3.7 mi. [6 km.] wide.
The city of Corinth was strategically located to "guard/monitor" the Isthmus—the only way from mainland Greece to the Peloponnese had to pass through it—think tolls and tariffs! In addition, it controlled the ancient "Diolkos" that connected the Saronic Gulf with the Gulf of Corinth. Ships and goods were transported along the Diolkos that followed a path somewhat close to where the modern canal is located.
The Diolkos [Greek meaning “haul across”] was a paved “road” that connected the Corinthian and Saronic gulfs before the Corinthian Canal was dug. It was built because sailing around the southern tip of the Peloponnese was very treacherous. Strabo, for example, writes ‘But when you sail around Cape Malea, forget your home” (= “you’ll never return!”; viii 6, 20)
For a brief description of the diolkos Click Here.