Massive fortification activity was conducted in the Caucasus during the reign of Kavad I (r. 488–496, 498–531– ) and later his son Khusrow I (r. 531–579– ), in response to the pressure by people in the north, such as the Alans. Key components of this defensive system were the strategic passes Darial in the Central Caucasus and Derbent just west of the Caspian Sea, the only two practicable crossing of the Caucasus ridge through which the land traffic between the Eurasian Steppe and the Middle East was conducted. A formal system of rulership was also created in the region by Khusrow I, and the fortifications were assigned to local rulers.