From A Feast for Crows (don't know what page, as this is from my Kindle version): Maester Luwin had taught them about Braavos, but Arya had forgotten much of what he'd said. It was a flat city, she could see that even from afar, not like King's Landing on its three high hills. The only hills here were the ones that men had raised of brick and granite, bronze and marble. Something else was missing as well, though it took her a few moments to realize what it was. The city has no walls. But when she said as much to Denyo, he laughed at her. "Our walls are made of wood and painted purple," he told her. "Our galleys are our walls. We need no other." In the Persian wars, there was a prophecy about the "wooden walls" of Athens protecting the city. But a politician/general named Themistocles said that the wooden walls that would protect the city would be a (wooden) fleet. He used pretty much the entire city's funds to build his fleet, and while the famed Three Hundred slowed the Persian army at Thermopylae, Themistocles' navy defeated the Persians, saving Athens (and most of Greece)--and fulfilling the prophecy.
