Alphen aan den Rijn is a town and municipality in the western Netherlands, in the province of South Holland, between Leiden and Utrecht. The town is situated on the banks of the river Oude Rijn (Old Rhine), where the river Gouwe branches off. The municipality had a population of 72,674 in 2010, and covers an area of 57.68 km2 (22.27 sq mi) of which 2.61 km2 (1.01 sq mi) is water. It also contains the remnants of a Roman castellum.