Kabul is located high up in a narrow valley between the Hindu Kush mountains, with an elevation of making it one of the highest capitals in the world. The city is said to be over 3,500 years old, mentioned since at least the time of the Achaemenid Empire. Located at crossroads in Asia - roughly halfway between Istanbul in the west and Hanoi in the east - it is in a strategic location along the trade routes of South and Central Asia, and a key location of the ancient Silk Road. It has been part of the Achaemenids followed by the Seleucids, Mauryans, Kushans, Kabul Shahis, Saffarids, Samanids, Ghaznavids, Ghurids, Khwarazmians, Qarlughids, Khaljis, Timurids, Mughals, and Hotaks, until finally becoming part of the Durrani Empire (also known as the "Afghan Empire") in 1747. Kabul became the capital of Afghanistan in 1776, during the reign of Timur Shah Durrani, the son of Ahmad Shah Durrani.