Plovdiv is the second-largest city in Bulgaria, and one of the oldest cities in Europe. It is located on six hills in the large plain between the Rhodope Mountains in the south of Bulgaria and the Balkan Range. The Maritsa River flows through the city on its way southeast before forming the Greek/Turkey border to the Aegean Sea. Between the narrow streets of its old town you will find the Ancient Theater and a lot of buildings of houses from 18th and 19th centuries.