Yerevan is the capital and largest city of Armenia and one of the oldest continuously inhabited cities in the world. The city is also the seat of the Araratian Pontifical Diocese, the largest diocese of the Armenian Apostolic Church and one of the oldest dioceses in the world. The history of Yerevan dates back to the 8th century BC, when King Argis I of Urartu founded the fortress of Erebuni on the western edge of the Ararat plain in 782 BC. Erebuni was planned as a great administrative and religious center, a fully royal capital. By the end of the ancient Armenian Kingdom, new capitals had been established and Yerevan's importance declined. After World War I, Yerevan became the capital. The city expanded rapidly in the 20th century, while Armenia was part of the Soviet Union. In a few decades, Yerevan went from being a provincial city of the Russian Empire to the main cultural, artistic and industrial canter of Armenia, as well as the seat of the national government. Among Yerevan's landmarks, Erebuni Fortress is considered the birthplace of the city, Katoghike Tsiranavor Church is the oldest surviving church in Yerevan, St. Gregory's Cathedral is the largest Armenian cathedral in the world, and Tsitsernakaberd is the official memorial to the victims of the Armenian Genocide. The city is home to numerous opera houses, theatres, museums, libraries and other cultural institutions. The Yerevan Opera Theatre is the main attraction of the Armenian capital, the National Gallery of Armenia is Armenia's largest art museum, sharing a building with the History Museum of Armenia, and the Matenadaran repository contains one of the world's largest repositories of ancient books and manuscripts.