The centuries from 11th to 15th centuries marked the flourishing of the Islamic arts in Iran, one of the best manifestations of which is the Mausoleum of Oljaytu, in Soltaniyeh city of Zanjan Province. Boasting a double-shelled dome and exquisite materials and themes used in the interior decoration, the mausoleum is a masterpiece of its kind from the 14th century. It represents the developments of arts and architectural techniques which characterize the innovations of the Ilkhanid Period in Iran and set the point for further developments. The mausoleum was recognized as a UNESCO World Heritage Site in 2005.