The Romano-Germanic Museum was built in 1974 in the immediate vicinity of the cathedral on the walls of an ancient Roman villa from the third century. This villa was discovered in 1941 during the construction of an air-raid shelter.
The main room of the villa was decorated with the beautiful Dionysus mosaic (around 220/230 A.D). During the construction of the museum the new building was erected around and above the ancient villa. So the mosaic was not relocated into the museum but sill lies at its original location.