Since the end of the 3rd century B.C. the nomads began to press the northern boundaries of Greco-Bactria. According to the archeological data defensive structures were built to protect the mountain passes at Derbent: one near to the river of Shurob at the Iron Gate, and another (Qapchigai-tepa) at the exit from the canyon of the Machai river. Nevertheless, in the second half of the 2nd century the nomadic tribes of Yuech-chi came round the Derbent passes from the east and west and invaded Greco-Bactria. Later, in the first centuries A.D. the rise of the Yuech-chi clan of Kushans led to the formation of the powerful Kushan empire including Bactria, today’s Afghanistan, Pakistan and northern India. Boysun became a country of Kushan fortresses whose powerful rammed earth walls have been preserved in ruins: Shurob-tepa, Uzundara, Machaikurgan, Munchak-tepa, Qalai Bolo and Saryband. The largest fortress was discovered at the place of old Greco-Macedonian fortress of Poenkurgan near Boysun town. In the 1st – 2nd centuries A.D. the strategic canyon at Derbend was blocked with a wall 1.5 km in length and 6 m in thickness, built of adobe bricks with stone and earth filling. The wall became the northern Kushan border. The line of this wall is clearly visible today. When at the end of the 2nd century the Kushan empire had come to decline, the territory of Boysun was deserted and only in the Bandykhan oasis the fortress and Saryband settlement continued to operate in the Kushano-Sassanian period up to the mid-4th century.