History of the Peloponnesian War

For to the navy he paid the greatest attention; seeing, I suppose, that the approach of the king's forces against them was easier by sea than by land: and he considered the Piraeus more serviceable than the upper city, and often advised the Athenians, in case of their ever being hard pressed by land, to go down into it, and defy the world with their navy.

Thus then the Athenians were enclosed with walls, and began to furnish themselves with other buildings immediately after the retreat of the Medes.