Mahabharata 12.87.5

धन्वदुर्गं महीदुर्गं गिरिदुर्गं तथैव चमनुष्यदुर्गमब्दुर्गं वनदुर्गं च तानि षट्

dhanvadurgaṃ mahīdurgaṃ giridurgaṃ tathaiva camanuṣyadurgamabdurgaṃ vanadurgaṃ ca tāni ṣaṭ

Our plain-language meaning of this verse is waiting for scholar review.