Kautilya's Arthashastra 7.10.30

7.10.30

nadīdurgaṃ tu dviguṇakleśakaram, udakaṃ ca pātavyaṃ vṛttikaraṃ cāmitrasya //

Of two fortified kings, one who has his forts on a plain is more easily reduced than the other owning a fort in the centre of a river; for a fort in a plain can be easily assailed, destroyed or captured along with the enemy in it, whereas a fort, surrounded by a river requires twice as much effort to capture and supplies the enemy with water and other necessaries of life.