Under leaders such as the second governor Gazi Husrev-beg, Sarajevo grew at a rapid rate. Husrev-beg greatly shaped the physical city, as most of what is now the Old Town was built during his reign. Sarajevo became known for its large marketplace and numerous mosques, which by the middle of the 16th century numbered more than 100. At the peak of the empire, Sarajevo was the biggest and most important Ottoman city in the Balkans after Istanbul. By 1660, the population of Sarajevo was estimated to be over 80,000. By contrast, Belgrade in 1683 had 100,000, and Zagreb as late as 1851 had 14,000 people. As political conditions changed, Sarajevo became the site of warfare.