Alphen aan en Rijn (population: 70,927 in 2006) 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 covers an area of 57.68 km² (22.27 mile²) of which 2.61 km² (1.01 mile²) is water. It also contains the remnants of a Roman castellum.