Now a museum, the castle's site was originally the seat for the archbishops of Trier, and was built in the mid 1200s. Like most, it fell into ruin, and was burned down by invading French in 1688. What remained was used as a quarry afterwards.
After trading hands a bit, it was given to King Friedrich Wilhelm IV of Prussia in 1823, who had it rebuilt in a pseudo-gothic style by a man named Karl Friedrich Schinkel...until he died and someone else finished it. Like most castles, this structure is very different in its layout and style than the original.
Friedrich Wilhelm moved in during Sept. 1842.
The castle is now used as a museum for life in the 1800s since the last person to live there left it in 1861. Prussia used it as a HQ until 1918. I'm not sure what happened to it exactly from there.
Anyways, there are a few interesting stories about the castle. One tells about a brother and sister, Othmar and Willeswind. While Othmar was off at war, a robber knight in the guise of a pilgrim-beggar came to the palace. Having been permitted entrance and thus knowledge of the defenses and the beauty of Willeswind, he returned a few days later, no longer in disguise, and demanded that Willeswind marry him or watch her castle and people suffer the attack he and his cronies would make. She refused of course and fled to a convent, but the knight ambushed her along the way, taking and locking her and her maid in a tower in the forest from which there was no escape. Lucky for them, Willeswind's raven, trained to pick berries as a lark originally, came in very handy, his one skill keeping them from dying for hunger or thirst for the 6 days they were in the tower. On that last day, it just so happened that Othmar came riding by and, being called over by Willeswind, killed the knight who had kidnapped her, saving the day. Honor was given to the raven as well, who had a statue built of him commemorating his loyalty.
The other story I found tells of a time when the lord of the castle went away for some reason, leaving his trusted steward in charge. The steward, however, had recently become obsessed with alchemy and began to use the gold entrusted to him in a series of failing experiments. Before long, his stupidity had bankrupted his lordship. The steward's daughter, Mina, was horrified and upset for she did not want her beloved father disgraced, but then when a mysterious pilgrim who happened to be a great alchemist turned up at the castle things began to look up. However, this "great alchemist" was, like all alchemists, a fraud and though promising to help the steward recover the lost fortune, continued to fleece him until there was no gold left at all in the treasury. The swindler then told the steward that a pure maiden's blood was as good as gold (what he's gonna get out of maiden's blood I really don't wanna know). The steward himself refused to give his daughter up, but Mina was conviniently eavesdropping and came separately to the 'alchemist' and offered herself as a willing sacrifice. He told her that'd be great, and to come on a certain day when the stars would be right.
Meanwhile, the Lord of the castle came home and one of his followers saw Mina (for the first time? Not very observant is he. Either that or he's a newbie). As in all fairy tales, the guy fell for her, so he got pretty excited the next night when he heard her come down the hall towards his room, but confused when she went past it and into the laboratory. He listened as she and the alchemist talked, her hoping there was another way, he saying there wasn't, and pulling out a knife. Just as the alchemist prepared to stab Mina (I really wonder how he thought he'd get away with this since he's -in- a castle and I would assume there are like, people who'd notice she's missing), the love-stricken boy jumped in and saved her. They lived happily ever after of course, and the lord of the castle even forgave the Steward since he was so sincere about his regret. ...Yeah.
You gotta love 1895 renditions of fairy tales. By the way, the place I'm getting most of these is a collection of tales I found on Google Books, http://books.google.com/books?id=hiYWAAAAYAAJ&pg=PA207&lpg=PA207&dq=Schonberg+Castle+Rhine&source=web&ots=gvaoPBjC--&sig=QO3fsbySaUaO9a5VPKsEyqNRi-0&hl=en&sa=X&oi=book_result&resnum=1&ct=result#PPR7,M1
Probably very cleaned up and polished to reflect Victorian morality, but still pretty amusing.