The Kalyan Mosque is Bukhara's congregational mosque or Friday Mosque. It was built in the 16th century on the site of an older mosque destroyed by Genghis Khan. The Kalyan minaret or Minara-i Kalan (Pesian/Tajik for the 'Grand Minaret') is part of the Po-i-Kalyan mosque complex and was designed by Bako and built by the Qarakhanid ruler Arslan Khan in 1127. The minaret is made in the form of a circular-pillar brick tower, narrowing upwards, with a diameter of 9m (30ft) at the bottom, 6m (20ft) at the top and a height of 46m (150ft) high. The Kalyan Minaret is also known as the 'Tower of Death', as for centuries criminals were executed by being tossed off the top. 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.