The Herzogskasten, literally the "Duke's Box", is a 13th-century ducal building and the oldest secular structure in Ingolstadt. Built around 1255, it began as part of the city's early castle complex and later served administrative and storage roles.
Today it houses the city library, so the interest is mainly the exterior and the way the medieval shell sits inside the old town. It is an easy stop while walking between Ingolstadt's churches, gates, and squares.