Syracuse in Sicily has been suggested as the model for Plato’s Atlantis by Gunnar Rudberg among others. The ancient city was built in a natural harbour with a large island, Ortygia, in it. A causeway bridged a strait between the island and the mainland. There was a wall around Ortygia and around its central citadel. The mainland part of the city was also walled.
Syracuse was founded around 743 BC by Archia, a Corinthian and over succeeding centuries developed into a major Mediterranean power, defeating the Carthaginians in 480 BC. In 413 BC the Athenian navy, one of the largest ever, was destroyed by the Syracusans. This happened when Plato was just a teenager and no-doubt it would have left a lasting impression on him.
After the death of Socrates, Plato travelled throughout the Eastern Mediterranean. While in Syracuse, he was highly critical of the morality of the city and in doing so he angered its ruler Dionysius I. This resulted in his being sold as a slave but fortunately his owner freed him. He then returned home to found his famous Academy. However, in later life, as his fame spread, Plato was enticed back to Syracuse to tutor the new ruler, Dionysius II.
To what extent his Sicilian experiences have had an influence on his writings is impossible to say. The idea that Plato was inspired by the layout of Syracuse to produce a more embellished and schematic version of it in his description of Atlantis is plausible, but no more than that.
What is virtually certain is that Syracuse was not a port prior to the de-glaciation following the last Ice Age as it would have been too far inland due to lower sea levels.