Aalborg (pronunciation IPA IPA: [ˈʌlb̥ɒːˀ]) is the fourth-largest city in Denmark - after Copenhagen, Aarhus and Odense - with an urban population of 121,540 (2006). The earliest settlements date back to around 700 AD. Its location by the Limfjord made it an important harbour during the Middle Ages and an industrial centre later. Today the city is in a transition from a working-class, industrial city to a knowledge-based one. One of the driving forces in the transition is the University of Aalborg, founded in 1974. It is the location of Aalborg Air Base, an important base of the Danish air force, and is the seat of a bishop within the Danish National Church.