Kautilya's Arthashastra 7.10.32

7.10.32

nadīdurgaṃ hi hastistambhasaṃkramasetubandhanaubhiḥ sādhyam anityagāmbhīryam avasrāvy udakaṃ ca //

Of two kings, one owning a fort surrounded by a river, and another having mountainous fortifications, seizing the former's land is better, for a fort in the centre of a river can be assailed by a bridge formed of elephants made to stand in a row in the river or by wooden bridges, or by means of boats; and the river will not always be deep and can be emptied of its water, whereas a fort on a mountain is of a self-defensive nature, and not easy to besiege or to ascend; and when one portion of the army defending it is routed out, the other portions can escape unhurt and such a fort is of immense service, as it affords facilities to throw down heaps of stone and trees over the enemy.