Meersburg Castle

Meersburg Castle, also known as the Alte Burg, in Meersburg on Lake Constance in Baden-Württemberg, Germany, is considered to be one of the oldest inhabited castles in Germany. The central tower was first built during the 7th century, though the original structure is no longer visible. Burg Meersburg is known as the old castle, in the reference to the neighboring 18th century New Castle.

Source: Wikipedia