History of the Peloponnesian War

For principally he was addicted to the sea because, as I think, he had observed that the forces of the king had easier access to invade them by sea than by land, and thought that Piraeus was more profitable than the city above. And oftentimes he would exhort the Athenians that, in case they were oppressed by land, they should go down thither and with their galleys make resistance against what enemy soever.

Thus the Athenians built their walls, and fitted themselves in other kinds, immediately upon the departure of the Persians.