Bukhara was founded in 500 BCE in the area now called the Ark. However, the Bukhara oasis had been inhabitated long before. The city has been one of the main centres of Persian civilization from its early days in 6th century BCE. From the 6th century CE, Turkic speakers gradually moved in. Bukhara's architecture and archaeological sites form one of the pillars of Central Asian history and art. The region of Bukhara was for a long period a part of the Persian Empire. The origin of its inhabitants goes back to the period of Aryan immigration into the region.