Arthashastra
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अतिदुर्ग प्रतिष्टब्धं वा सामन्त आटविक तत् कुलीन अपरुद्धानाम् अन्यतमेन अवग्राहयेत् ॥
If he has entrenched himself in an exceptionally inaccessible fortress, he should be besieged and captured by employing either a neighboring ruler, a forest chieftain, a member of his own family, or an exiled prince.
english translation
Chapter 3
Verse 23
Verse 25