Kautilya classifies six types of Durga (fort): Audaka (water), Parvata (mountain), Dhanvana (desert), Vana (forest), Mahi (earthen), and Nri (human/defended by men). This chapter explores fortress design principles, siege tactics (Avarodha), when to attack vs. negotiate, and most importantly - just conquest (Dharma Vijaya). How to treat conquered populations, when to preserve local leadership, and why oppressive conquest ultimately fails.