Why did Henry IV build royal castles in the Harz Mountains?

Henry IV, Holy Roman emperor, built royal castles in the Harz Mountains primarily to consolidate and protect his royal power and possessions. During his minority, royal lands and rights had been widely ceded or appropriated by the nobility, particularly in Saxony.12

Upon coming of age, Henry sought to recover these holdings and establish a secure royal territory where he and his court could reside. By fortifying this region under the supervision of his ministeriales—a class of unfree knights directly responsible to the crown—he aimed to bypass the military reliance on independent lay princes, whose influence he sought to diminish. This strategy allowed the king to maintain a base of power independent of the local magnates, though the ruthless repossession of rights and the use of foreign officials to manage these castles ultimately incited a major rebellion among the Saxon nobility and peasantry.21