Heidelberg is a German city situated in the south-west of the country. The fifth largest city of the German State of Baden-Württemberg after Stuttgart, Mannheim, Karlsruhe and Freiburg im Breisgau, Heidelberg is part of a densely populated region known as the Rhine-Neckar Metropolitan Region. As of 2009, over 145,000 people live within the city's area. Heidelberg lies on the River Neckar in a steep valley in the Odenwald. Heidelberg — having been a former residence of the Electoral Palatinate in the past — is the seat of the University of Heidelberg, which is well-known far beyond its and Germany's borders. Heidelberg is also a popular tourist destination due to its romantic and picturesque cityscape's character, including the Heidelberg Castle and the baroque style Old Town. The US Army has had a military base in Heidelberg since 1951.