Tübingen (ˈtyːbɪŋən; Dibenga) is a traditional university city in central Baden-Württemberg, Germany. It is situated south of the state capital, Stuttgart, and developed on both sides of the Neckar and Ammer rivers. about one in three of the 90,000 people living in Tübingen is a student. As of the 2018/2019 winter semester, 27,665 students attend the Eberhard Karls University of Tübingen. The city has the lowest median age in Germany, in part due to its status as a university city. As of December 31, 2015, the average age of a citizen of Tübingen is 39.1 years. Immediately north of the city lies the Schönbuch, a densely wooded nature park. The Swabian Alb mountains rise about (beeline Tübingen City to Roßberg - 869 m) to the southeast of Tübingen. The Ammer and Steinlach rivers are tributaries of the Neckar river, which flows in an easterly direction through the city, just south of the medieval old town. Large parts of the city are hilly, with the Schlossberg and the Österberg in the city centre and the Schnarrenberg and Herrlesberg, among others, rising immediately adjacent to the inner city. The highest point is at about above sea level near Bebenhausen in the Schönbuch forest, while the lowest point is in the city's eastern Neckar valley. The geographical centre of the state of Baden-Württemberg is in a small forest called Elysium, near the Botanical Gardens of the city's university. The area was probably first settled by ancient humans in the 12th millennium BC. The Romans left some traces here in AD 85, when they built a limes frontier wall at the Neckar River. Tübingen dates from the 6th or 7th century, when the region was populated by the Alamanni people. Some historians argue that the Battle of Solicinium was fought at Spitzberg, a mountain in Tübingen, in AD 367, although there is no evidence for this. Tübingen first appears in official records in 1191. The local castle, Hohentübingen, has records going back to 1078, when it was besieged by Henry IV, king of Germany. Its name was transcribed in Medieval Latin as Tuingia and Twingia.