Lenzburg is a municipality in the district of Lenzburg in the canton of Aargau, Switzerland. It is the capital city of the district. The town was founded in the Middle Ages. It is in the Seetal valley, about south of the Aare delta. Lenzburg and the municipalities of Niederlenz and Staufen have merged over time. The city name refers to the name of the castle which can be translated as "spring castle".