Mahabharata 12.87.5

12.87.5

dhanvadurgaṃ mahīdurgaṃ giridurgaṃ tathaiva ca |
manuṣyadurgamabdurgaṃ vanadurgaṃ ca tāni ṣaṭ || 5 ||

"Bhishma said, 'It is proper, O Bharata, to enquire about the conduct that should be followed and the defences that should be adopted with respect to the city in which, O son of Kunti, a king should reside. I shall, therefore, discourse to thee on the subject, referring especially to the defences of citadels. Having listened to me, thou shouldst make the arrangements required and conduct thyself attentively as directed. Keeping his eye on the six different kinds of citadels, the king should build his cities containing every kind of affluence and every other article of use in abundance. Those six varieties are water-citadels, earth-citadels, hill-citadels, human-citadels, mud-citadels, and forest-citadels.