Duisburg is a German city in the western part of the Ruhr Area in North Rhine-Westphalia. It is an independent metropolitan borough within Regierungsbezirk Düsseldorf. With the worlds biggest inland harbor and its proximity to Düsseldorf Airport, Duisburg has become an important venue for commerce and steel production. In the early Middle Ages it was a royal court of the Franks, first mentioned in writing in 883. Like many other cities, todays Duisburg is a result of numerous incorporations of surrounding towns and smaller cities. It is the fifteenth-largest city in Germany and the fifth-largest city in North Rhine-Westphalia with 486,855 residents at the end of 2013. The city is renowned for its steel industry. The last remaining coal mine closed down in the summer of 2009, but Duisburg had never been a coal-mining center to the same extent as other places in the Ruhr region. All blast furnaces in the Ruhr are now located in Duisburg. In 2000 49% of all hot metal and 34.4% of all pig iron in Germany was produced here. It also has a large brewery, the König Brauerei, located in Duisburg-Beeck, which makes the König Pilsener brand. The University of Duisburg-Essen, with 39,000 students, ranks among the 10 largest German universities.