Another war term from Dr. Kenneth J. Hammonds accounts of the Three Kingdoms period in China. General Zhuge Liang, our hero from Straw Boats Borrow Arrows, was trapped in a city without his main army. Rather than surrendering to his enemy, or fortifying for attack, he threw open the gates and played chess on the wall (another account has him playing the zither). His enemy, Sima Yi, known to be a shrewd strategist, approached the fort and noticed the calm confidence of Zhuge Liang and the townspeople. He assumed that the fort must be formidably defended for its inhabitants to be so nonchalant, and Sima Yi pulled his men back in concern.
According to Wikipedia, the Empty Fort strategy is the thirty-second of the thirty-six stratagems of war. This is an essay found in the Book of Qi written by Xiao Zixian during the Liang Dynasty.