Deriving its name from Horsteberg (a church and cloister on the right bank of the river Aa built by Frisian Liudger, a missionary sent by Charlemagne in 793 to onvert Saxons with whom he'd been battling) Münster, is one of the 42 agglomeration areas and one of the biggest cities of Germany in terms of acreage. However, this includes substantial sparsely populated, agrarian districts which were formerly separate local government areas but were amalgamated in 1975. Thus nearly half the city's area is agricultural, resulting in a low population density of approximately 900 inhabitants per km². Münster gained the status of a Großstadt (major city) with more than 100,000 inhabitants in 1915. Currently there are around 270,000 people living in the city, with about 48,500 students, only some of whom are recorded in the official population statistics as having their primary residence in Münster.