Chapter: The Nine Situations 11.9
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Ground which is reached through narrow gorges, and from which we can only retire by tortuous paths, so that a small number of the enemy would suffice to crush a large body of our men: this is hemmed in ground.

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Places where there is limited access make great fortresses, although it is also possible you may be trapped there.

In a narrow corridor, troops may be faced only by an equal number. If additional defenses can be added, such as overlooking high ground, then this can become impregnable.