The Castle Wespenstein is located in Thuringia, Germany, and was built around 1250. Due to a fire in the 17th century, most of the roofs were damaged and a large area of the castle was abandoned. The rubble was deposited behind the walls which were not supposed to carry any earth pressure. In spite of that, the vast majority of the walls remained stable over the centuries. In one particular section, a sudden collapse of the backfilled wall took place in 2010. This contribution gives an overview of the wall and backfill mechanical properties. A geotechnical analysis of the wall and backfill system is performed and options for the rehabilitation are discussed.