Arnstadt is a town in Ilm-Kreis, Thuringia, Germany, on the river Gera about 20 kilometres (12 mi) south of Erfurt, the capital of Thuringia. Arnstadt is one of the oldest towns in Thuringia, and has a well-preserved historic centre with a partially preserved town wall. The town is nicknamed Das Tor zum Thüringer Wald because of its location on the northern edge of that forest. Arnstadt has a population of approximately 27,000.