The Ritterbruder is a unit of the knightly cavalry class, unique to the Teutonic Order and available from the 14th Century onwards. It is definitely the most powerful, but also the slowest cavalry unit in Age of Chivalry Hegemony. Therefore, Ritterbrüder are not really suited for situations where getting backup to the scene as soon as possible is crucial.
Type: Knightly Cavalry
Available To: Teutonic Order
Trained at: Castle
Cost: 90 Food, 90 Gold
Armor/Pierce Armor: 4/3
Special: +5 vs. Siege Units; gets -5 hit points from Popular Assembly
Attack: Forging, Iron Casting, Blast Furnace, Chivalric Order, Imperial Privilege (Teutonic Order Unique Technology)
Armour: Scale Barding Armour. Mail Barding Armour, Plate Barding Armour, Cistercian Reform (Teutonic Order Unique Technology)
Hit Points: Bloodlines
The Ritterbruder is definitely and by far the most powerful cavalry unit. At its full strength this unit will be packed with armour, ready to deal great damage and definitely a competitor to the Wapentuer when dealing with any kind of infantry. It may be slow, but it's a bruiser.
Watch out for archers, though: the Ritterbruder's heaviness and low speed make it vulnerable to ranged units, especially the Lithuanian Bajoras, but also heavier Lancers, like the Reichsritter, can cause severe damage--and are more likely to appear in greater numbers, than the quite expensive Ritterbruder. They're also vulnerable to Priests, since Saxony does not receive Heresy.
Knights, many from the non-noble ministeriales social class, travelled to the east to fight against the pagans on behalf of the Teutonic Order. These Ritterbrüder made up the knightly force of the Order.