Does anyone know something about Castle Drakenfel?

You mean Castle Drachenfels in the Grey Mountains?

Once home to the infamous Count Drachenfels (Yeovil's excellent book of same name though it departs a bit from warhamme canon in some regards).

There is an adventure about Castle Drachenfels from 1st Edition, which is derived from the novelisation valvorik posted. Game stores may have it if they have archival stuff, or you can usually find it at online auction or some other chicanery.

Castle Drachenfels: Dark adventure in the Grim Fortress of the Great Enchanter
by Carl Sargent

Warhammer Fantasy Roleplay sourcebook and adventure location, with scenario ideas and some new rules. From the back cover: Deep within the Grey Mountains, almost hidden amongst the jagged peaks, lurks the dark imposing castle of the Great Enchanter Drachenfels. Even though its master is now dead, his terrible presence scoured from the conscious mind and only mentioned in fireside tales, the mute stone walls of the castle still seem to stir with dark and malicious purpose. Many brave Adventurers have entered the castle's forbidding walls in search of treasure and glory, or to satisfy their morbid curiosities. Few have ever returned. Those that have tell of strange forces and long-dead creatures prowling the slimy corridors, guarding the bowels of their master's domain from unwelcome intrusion. Now you stand awestruck at the gaping maw that is the castle's entrance. Now you must enter, and test your mettle against the dangers within... Castle Drachenfels is a 112 page Warhammer Fantasy Roleplay adventure for experienced adventurers based on the locations and characters from the Warhammer novel 'Drachenfels' by Jack Yeovil. In addition to a history of Constant Drachenfels and his castle, this book also contains detailed notes on adventuring in the dank, dark corridors of Castle Drachenfels, a selection of adventure outlines and plots, a party of pregenerated characters, new monsters, and new spells and magic items. Packed with dark and mysterious locations to explore and rich treasures to reap, Castle Drachenfels opens up a whole new world to anyone who plays Warhammer Fantasy Roleplay. Dare you enter the castle gates and see what lies beyond...

NOVEL: Drachenfels: A Genevieve Novel (Warhammer)
"No review can truly due this book justice. Drachenfels is without a doubt the best Black Library novel available, and is one of the best books I have ever read. Unlike other Warhammer novels Drachenfels is brimming with humour and satire. I was delighted to find myself laughing aloud to a Black Library release (which are normally filled with graphic violence). The characters are nothing short of incredible and are unbelievably realistic. Jack Yeovil (Kim Newman) does an excellent job of fleshing out the warhammer universe, and you'll be suprised by the number of famous warhammer names within this novel. Black Library has done a great job of reprinting this marvelous book. I am fortunate enough to own one of the original Genevieve short stories, and without this reprint I might never have been able to find this fantastic book. BUY THIS BOOK- You will not regret it. "

Here are some details of Castle and the Drachenfels himself: Warhammer Lexicanum - Castle Drachenfels

They are spoilers for the story, which I also recommend reading. It is best Warhammer stories written.

I've got the old Castle Drachenfels WFRP supplement. If you have any specific questions, I might be able to answer them. The nutshell version of the Castle is "It's an old castle possessed by the spirit of its most notorious tenant. If you go in, you probably won't ever come out again. If you do come out, you'll be broken and insane for the rest of your life." It's a horror novel as roleplaying adventure.