Niš is the third largest city of Serbia and the administrative center of the Nišava District in southern Serbia. According to the 2011 census, the city has population of 183,164, while urban area of Niš has 187,544 inhabitants; the administrative area has a population of 260,237. It is one of the oldest cities in the Balkans and Europe, and has from ancient times been considered a gateway between the East and the West. It was founded by the Scordisci in 279 BC, after an invasion of the Balkans. The city was among several taken in the Roman conquest in 75 BC; the Romans built the Via Militaris in the 1st century, with Naissus being one of its key towns; it is also the birthplace of Constantine the Great, the first Christian Emperor and the founder of Constantinople, and Constantius III and Justin I. It is home to one of the oldest churches in Serbia, dating to the 4th century, located in the suburb of Mediana. The Balkans came under the Eastern Roman Empire. In the 6th century, Slavs started settling the Balkans, while the town was held by the Byzantines until the 9th century, when it came under Bulgar rule. The town switched hands between the two, before being given by the Byzantines to the Serbs in the 12th century. Niš served as Stefan Nemanjas capital. It was conquered by the Ottomans in the 15th century, becoming the seat of a sanjak initially in Rumelia Eyalet, laterly in Niš Eyalet and finally in Danube Vilayet. It was liberated by the Serbian Army in 1878 during the Serbian–Ottoman War . Niš is one of the most important industrial centers in Serbia, a center of electronics industry, industry of mechanical engineering, textile and tobacco industry. Constantine the Great Airport is its international airport. In 2013 the city was host to the celebration of 1700 years of Constantines Edict of Milan.