Kautilya's Arthashastra 2.3.1

2.3.1

caturdiśaṃ janapadānte sāmparāyikaṃ daivakṛtaṃ durgaṃ kārayet, antardvīpaṃ sthalaṃ vā nimnāvaruddham audakam, prāstaraṃ guhāṃ vā pārvatam, nirudakastambam iriṇaṃ vā dhānvanam, khañjanodakaṃ stambagahanaṃ vā vanadurgam //

ON all the four quarters of the boundaries of the kingdom, defensive fortifications against an enemy in war shall be constructed on grounds best fitted for the purpose: a water-fortification (audaka) such as an island in the midst of a river, or a plain surrounded by low ground; a mountainous fortification (párvata) such as a rocky tract or a cave; a desert (dhánvana) such as a wild tract devoid of water and overgrown with thicket growing in barren soil; or a forest fortification (vanadurga) full of wagtail (khajana), water and thickets.