Trondheim is a city and municipality in the county of Sor-Trondelag, Norway and is the center of education, technical and medical research, with close to 30,000 students. Trondheim is the country's third largest city, with an population of approximately 159,000 people. Trondheim is located where the Nidelva River meets a large fjord, and is the center of the Trondheim region. Nidaros Cathedral is the northernmost Medieval cathedral in the world; and the synagogue is among the most northern in the world. Trondelag Teater, the oldest theater in Northern Europe that is still in use, is also located in Trondheim. Landmarks or tourist interests include Kristainsten Fortress built in 1681; Munkholmen, a vacation island with a history as a prison; the Norwegian University of Science and Technology; Stiftsgarden, the royal residence; Sverresborg, medieval castle, and nearly a dozen museums. In addition to all the colleges, universities, and high schools, the Air Force Academy of the Royal Norwegian Air Force is also located in Trondheim. Trondheim has a broad music influence, and is known for rock, jazz, and classical music. There is a music conservatory and a municipal music school, as well as a symphony orchestra and singing school. Rock bands and artists include groups famous throughout the world, and a band named after the city, that comes from Scotland.