Plovdiv is the second-largest city in Bulgaria and an administrative center of Plovdiv County. It is situated in the Upper Thracian Plain, on both banks of the Maritsa river. Plovdiv is one of the oldest cities in Europe – the beginning of its history dates back to 4,000 years BC. In the beginning, the ancient settlement was situated on the natural elevation between Nebet, Taksim and Dzhambaz hills (the Three Hills). In ancient times Thracians inhabited the Three Hills and built a fortified settlement – the largest city in Thrace. In the 4th century BC, Plovdiv was conquered by Phillip of Macedonia. He gave the city one of its many names – Phillipopolis and had it surrounded by thick fortified walls. Later the Thracians regained their power over the city, but after a series of battles in the 1st century AD, it became a part of the Roman Empire.