from 1186, Pope Urban III refers to the city as Hafn, but this is probably just a shortened version of the full name. In around 1160, Valdemar I gave control of Copenhagen to Absalon, Bishop of Roskilde. Whereas other cities in the Danish realm were under the governance of the king, Copenhagen was given to the Bishop of Roskilde as its lord and master. In the years that followed, the town grew tenfold in size. Churches and abbeys were founded. Copenhagen's economy blossomed as a result of the hugely prosperous herring fishery trade, which provided large parts of Roman Catholic Europe with salted herring for Lent. Behind the new earthworks Copenhagen is located at the most important approach to the Baltic Sea and the rich North German trading towns of the Hanseatic League, providing it with power and wealth but also threatening its very existence. The city was fortified with a city wall of stone during the 13th century, and from about 1290 until the middle of the 19th century all traffic entering and leaving Copenhagen had to pass through one of Copenhagen's four city gates or the harbour. Although several Danish towns had ramparts at the time, the majority of them were earth ramparts possibly with palisades on top and a moat. Copenhagen was the second Danish town after Kalundborg to be fortified with a wall and towers. This is a strong indication that it was an important town at the time. The plague disaster known as the Black Death is likely to have taken place in the late summer of 1350. Time and again the town was besieged and laid waste by the Hanseatic League. At the same time the Danish king was also attempting to take Copenhagen back from the bishop. The crown succeeded in 1416, when Eric of Pomerania took over control of the town. Thereafter Copenhagen came under the rule of the Danish Crown. Despite centuries of power struggles and warring, the town grew increasingly rich. Copenhageners did a brisk trade with friend and foe alike. Foreign merchants came to the town. Craft guilds were established and in 1479 the University of Copenhagen was founded. In 1581 Christopher Valkendorf supervised the largest expansion of the ramparts in the history of the city, as with the invention of cannons vastly extended earth ramparts were required. Renaissance By the time of Christian IV's coronation in 1596, Copenhagen