After the Battle of Salamis, the Athenians came to Sparta to ask the Spartans to join in a military campaign to drive the Persians out of Attica. The Ephors took a several days deciding what to do. Herodotus speculates they wanted to make sure the wall across the Isthmus was complete before proceeding, because they suspected the Athenians would surrender to the Persians if the opportunity arose. The Ephors finally consented and sent a full contingent of five thousand Spartan warriors and thirty five thousand Helots. This army, along with the rest of the Greeks, went on to win the Battle of Platea.
I have not been able to find a second source describing the construction of this Peloponnesian Wall and specifically where it was located. Later, 450 A.D, a Byzantine Wall was built across the Isthmus. There are excavated ruins for this later wall.