Yerevan is one of the oldest continuously inhabited cities in the world. Its history dates back to the 8th century BC. It was then that King Argisthim I founded the fortress of Erebuni, which was to be a large administrative and religious center. In the Late Ancient Kingdom of Armenia, the importance of Yerevan gradually declined. After World War I, Yerevan became the capital of the First Republic of Armenia. It lies in the heart of the Armenian Highlands in the northeastern part of the Ararat Mountains in the valley of the Hrazdan River. Its area is 223 km². The number of inhabitants exceeded a million, which represents a third of the population of Armenia. It is the administrative, cultural and industrial center of the country. Yerevan is also an associate member of Eurocities. In 2012, UNESCO designated it as the world book capital.