An Ordensburg (plural in German: Ordensburgen) was a fortress built by crusading German military orders during the Middle Ages. The term "Ordensburgen" was also used during Nazi Germany to refer to training schools for Nazi leaders.
Hitlers ordensburgen 1 2
The Ordensburgen were originally constructed by the Livonian Brothers of the Sword and later the Teutonic Knights to fortify territory in Prussia and Livonia against the pagan aboriginals. Later, Ordensburgen were used to defend against Poland and Lithuania. The Ordensburgen often resembled cloisters. While they were considerably larger than those in the Holy Roman Empire, they were much scarcer in the Monastic state of the Teutonic Knights. While a normal castle in the Reich would control about 38 km2, a castle would control 370 km2 in Prussia and 789 km2 in Livonia, Courland and Estonia. The few small castles are considered to be of vassals, while the larger ones might have served as arsenals and strongholds against rebels and invaders.
Most Ordensburgen were rectangular, even quadratic in form, built from red brick and lacking a Bergfried. Many castles had no towers at all, as the bailey, a mighty quadrangle, was considered sufficient for defence. Note that many, possibly most of the castles in the list below are NOT Ordensburg either in terms of builder/possessor (held by vassals or bishops rather than Order) or architecture (although some bigger episcopal castles did copy Order architecture).