History of the Peloponnesian War

For Themistocles devoted himself particularly to the navy, because, as it seems to me, he had observed that the approach of the King's forces was easier by sea than by land; and he thought that the Peiraeus would prove more serviceable than the upper city, and often advised the Athenians, if ever they were hard pressed on land, to go down to the Peiraeus, and resist all their opponents with their fleet.

It was in this way, then, that the Athenians got their walls built, and came to be engaged upon their other fortifications, immediately after the withdrawal of the Persians.