The Abbasid Caliphate built the city of Baghdad along the Tigris in the 8th century as its capital, and the city became the leading metropolis of the Arab and Muslim world for five centuries. Baghdad was the largest multicultural city of the Middle Ages, peaking at a population of more than a million, and was the centre of learning during the Islamic Golden Age. The Mongols destroyed the city and burned its library during the siege of Baghdad in the 13th century.