The northern sections of what is now Serbia persisted as the Serbian Despotate, with Belgrade as its capital. The city flourished under Stefan Lazarević, the son of Serbian prince Lazar Hrebeljanović. Lazarević built a castle with a citadel and towers, of which only the Despot's tower and the west wall remain. He also refortified the city's ancient walls, allowing the Despotate to resist Ottoman conquest for almost 70 years. During this time, Belgrade was a haven for many Balkan peoples fleeing Ottoman rule, and is thought to have had a population ranging between 40,000 and 50,000 people.