Plovdiv is situated in a fertile region of south-central Bulgaria on the two banks of the Maritsa River. The city has historically developed on seven syenite hills, some of which are high. Because of these hills, Plovdiv is often referred to in Bulgaria as "The City of the Seven Hills". There is evidence of habitation in Plovdiv dating back to the 6th millennium BCE, when the first Neolithic settlements were established. The city was subsequently a local Thracian settlement, later being conquered and ruled also by Persians, Ancient Macedonians, Celts, Romans, Byzantines, Goths, Huns, Bulgarians (Thracians, Bulgars, Slavic tribes, etc.), Crusaders, and Ottoman turks.