Lenzburg Castle (German: Schloss Lenzburg) 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 (altitude: 504 m), which rises approximately 100 m over the surrounding plain but is only about 250 m in diameter. The oldest parts of the castle date to the 11th century, when the Counts of Lenzburg built it as their seat. The castle, its historical museum and the castle hill with its Neolithic burial grounds are listed as heritage sites of national significance.