The Art of War by active 6th century B.C. Sunzi

9. In dry, level country, take up an easily accessible position with

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rising ground to your right and on your rear, [Tu Mu quotes T’ai Kung as saying: "An army should have a stream or a marsh on its left, and a hill or tumulus on its right."] so that the danger may be in front, and safety lie behind. So much for campaigning in flat country.