When the Greeks sent to Delphi to ask how they were to defend themselves against Xerxes, who had invaded their country, the evasive answer given was to this effect -
Pallas hath urged and Zeus, the sire of all,
Hath safety promised in a wooden wall;
Seed-time and harvest, weeping sires shall tell
How thousands fought at Salamis and fell.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894
More on Wooden Wall from Fact Monster: