Der Luisenpark in Mannheim. Mannheim is the 3rd-largest city in Baden-Württemberg after Stuttgart and Karlsruhe with about 305,000 inhabitants. It’s located at the center of the larger, densely populated Rhein-Neckar Metropolitan Region with a population of 2.4 million. The Rhein separates Mannheim from Ludwigshafen in the neighboring state of Rheinland-Pfalz; the border with Hessen is just to the north. Mannheim is downstream along the Neckar river from the city of Heidelberg.