An imperial castle or Reichsburg was a castle built by order of the Holy Roman Emperor, whose management was entrusted to Reichsministeriales or Burgmannen. It is not possible to identify a clear distinction between imperial castles and the fortified imperial palaces or Pfalzen, because many imperial castles were used by German kings for temporary stays. A large number of imperial castles were built in regions such as Swabia, Franconia, the Palatinate and the Alsace, where there were a high density of imperial estates (Reichsgüter) during the Hohenstaufen era.