Canterbury is a historic English cathedral city and UNESCO World Heritage Site, which lies at the heart of the City of Canterbury, a local government district of Kent, England. It lies on the River Stour. The Archbishop of Canterbury is the primate of the Church of England and the worldwide Anglican Communion owing to the importance of St Augustine, who served as the apostle to the pagan Kingdom of Kent around the turn of the 7th century. The city's cathedral became a major focus of pilgrimage following the 1170 martyrdom of Thomas Becket. A journey of pilgrims to his shrine served as the frame for Geoffrey Chaucer's 14th century classic The Canterbury Tales. Canterbury is a popular tourist destination: consistently one of the most-visited cities in the United Kingdom, the city's economy is heavily reliant upon tourism. The city has been occupied since Paleolithic times and served as the capital of the Celtic Cantiaci and Jute Kingdom of Kent. Many historical structures fill the area, including a city wall founded in Roman times and rebuilt in the 14th century, the ruins of St Augustine's Abbey and a Norman castle, and the oldest extant school in the world, the King's School. Modern additions include the Marlowe Theatre and the St Lawrence Ground, home of the Kent County Cricket Club. There is also a substantial student population, brought about by the presence of the University of Kent, Canterbury Christ Church University, the University for the Creative Arts, and the Girne American University Canterbury campus. Canterbury remains, however, a relatively small city in terms of geographical size, when compared with other British cities.