Arthashastra
धान्वन वन दुर्ग निम्न दुर्गस्थं वा योग अग्नि धूमाभ्यां नाशयेयुः ॥
or they may destroy him by means of an explosive fire or poisonous snake when he has entrenched himself in a fort, in a desert, in a forest, or in a valley.
english translation
dhAnvana vana durga nimna durgasthaM vA yoga agni dhUmAbhyAM nAzayeyuH ॥
hk transliteration by Sanscript