Avdurga means a fortress abound in ap (water). That fortress will be beyond the reach of the enemies. This can be done by taking advantage of a natural river, or by creating an artificial canyon around the fortress, so that such water-animals as crocodiles can stay there. In his commentary, Rāghavācārya (Raghavacharya) has written — avdurgam agādhodakena nakrādiyuktena nadyādinā parito veṣṭitam. It is to be noted that Mahābhārata (Mahabharata) has not mentioned the name of avdurga among six kinds of fortresses. However, disturbed by repeated attacks from Jarāsandha (Jarasandha), Kṛṣṇa (Krishna) build up a city-fortress in Dvārakā (Dwaraka), surrounded by the sea. This could be called ‘avdurga’.