Kautilya's Arthashastra 2.3.2

2.3.2

teṣāṃ nadīparvatadurgaṃ janapadārakṣasthānam, dhānvanavanadurgam aṭavīsthānam āpady apasāro vā //

Of these, water and mountain fortifications are best suited to defend populous centres; and desert and forest fortifications are habitations in wilderness (atavísthánam).

Or with ready preparations for flight the king may have his fortified capital (stháníya) as the seat of his sovereignty (samudayásthánam) in the centre of his kingdom: in a locality naturally best fitted for the purpose, such as the bank of the confluence of rivers, a deep pool of perennial water, or of a lake or tank, a fort, circular, rectangular, or square in form, surrounded with an artificial canal of water, and connected with both land and water paths (may be constructed).