Nisaea was the main port of ancient Megara

Megara

Megara is an ancient city in Attica, Greece. It lies in the northern section of the Isthmus of Corinth opposite the island of Salamis, which belonged to Megara in archaic times, before being taken by Athens. Megara was one of the four districts of Attica, embodied in the four mythic sons of King...

.

According to Thucydides

Thucydides

Thucydides was a Greek historian and author from Alimos. His History of the Peloponnesian War recounts the 5th century BC war between Sparta and Athens to the year 411 BC...

 (4.66), the distance of the port from Megara itself was about eight Greek furlongs; Fraser says it is a mile and a half from the modern town. When Megara joined the Athenian alliance c. 459 BC, the Athenians built and garrisoned two long walls between Megara and Nisaea; in 424 the Megarians got possession of the walls and destroyed them, but in the fourth century they were rebuilt by Phocion

Phocion

Phocion was an Athenian statesman and strategos, and the subject of one of Plutarch's Parallel Lives....

 and were still extant in Strabo

Strabo

Strabo, also written Strabon was a Greek historian, geographer and philosopher.-Life:Strabo was born to an affluent family from Amaseia in Pontus , a city which he said was situated the approximate equivalent of 75 km from the Black Sea...

's time.

The hill of St. George on the eastern side of the harbor "appears to have been the acropolis of Nisaea mentioned by Pausanias."