Lenzburg Castle is a castle located above the old part of the town of Lenzburg in the Canton of Aargau, Switzerland. It ranks among the oldest and most important of Switzerland. The castle stands on the almost circular castle hill, which rises approximately 100 m (330 ft) over the surrounding plain but is only about 250 m (820 ft) in diameter. The oldest parts of the castle date to the 11th century, when the Counts of Lenzburg built it as their seat.