Burg Hengstenberg

Castle Hengstenberg is a rock castle, half built on and half hewn from a crag. It used to be a potent stronghold of a family of local importance, overlooking nearby towns and woods and several times proving to be nigh-unassailable.
Even with the invention of gunpowder weapons it was very hard to take Hengstenberg, mostly due to its position on a crag among rocky hills and outcroppings, which made it difficult to bring heavy artillery or erect earthworks, exposing the besiegers to fire from the castle. Hengstenberg was twice reinforced with additional outworks, but additional expansion was hampered by the very terrain that supported the castle's defensibility before, and advances in artillery eventually made Hengstenberg obsolete.
In 1031 Oldwig von Hengstenberg, last member of the formerly illustrious line, died without a male heir, and the castle was escheated to his liege, the emperor. Initially plans were drafted to use Hengstenberg as the emperor's retreat due to the largely untouched woodland and pleasantly-looking terrain, but they were abandoned, as it proved difficult and costly to bring the rock-hewn rooms to the level of comfort that suited a man of imperial rank.
Instead the former keep was eventually repurposed as a prison for important people of noble distinction - it still provided significant levels of protection from unwanted entry, if the would-be rescuers didn't bring heavy siege artillery, and commanded the surrounding lands, providing relatively good visibility and fields of fire for the defenders. And, naturally, if one tried to escape from a room hewn from the rocky depths, he would find such a task very daunting.
The first of the prisoners to come to Hengstenberg and one of the few to never leave it was the very man whose delicate case called for a relatively remote and very secure prison that the castle was destined to be from now on - emperor's own bastard son, whose mental malady made him wroth and irritable and compelled him to mistreat and eventually brutally murder his mistress, was interred in one of Hengstenberg's rooms within the eponymous crag and never saw the light of day again for the remainder of his life, slowly degenerating to a rather animalistic state and wasting away, dying in abject misery a few years later.
Rumours claim that this method of disposing of inadequate, infirm, or simply undesirable family members became rather popular with the subsequently elected emperors, but no definitive proof of that was obtained, and the castle is known to serve chiefly as a place where important hostages are treated with watchful hospitality until eventual release, with most inmates - invariably nobles of distinction or their kin - finding their tenure acceptable.
The most notable case is, perhaps, that of Hans Hermann von und zu Sommerfeldt. He was taken as a hostage by the emperor to ensure his father's compliance and entered Hengstenberg at the tender age of six. He was treated well, but his father, not lacking for children, eventually reneged on his promises to the emperor, utilizing local discontent and the dubious imperial legitimacy to impose harsh tolls on the traders and sometimes resorting to outright robbery to replenish his coffers, wisely surmising that the emperor would let him be at least temporarily, having other problems to deal with.
Hans spent the next ten years of his life in Hengstenberg, and one would reasonably surmise that such a trial in such a young age would be a great calamity for this unfortunate man. However, the emperor either took pity on him, or, more probably, saw a good opportunity to obtain a loyal vassal, and provided the young man with a good and comprehensive education, with some teachers permanently stationed in Hengstenberg and some - chiefly the professors of Emperor Wigbold's university - coming from time to time. When Hans Hermann reached majority, he was closely examined by learned men and found to be of considerable wit and skill, and some time afterwards he, still being tutored by eminent men, was transferred to live in the comfort of the imperial court at Isenburg, though he was still a hostage and a prisoner. He found imperial favour and was subsequently awarded the title that his father was forced to forfeit for his transgressions.
This case, however, is the exception rather than the rule, and while the treatment is decent, and the cells aren't entirely uncomfortable, prolonged terms at Hengstenberg are known to leave the inmates greatly distressed - most likely because of the seclusion they are forced to endure.