Under Oak Hill

From the introduction:

The castle of Eichenstein has been towering over Oak Hill for centuries, protecting and ruling the valley below. But now, long after the last of the von Eichensteins passed away and the castle fell into ruins. Lately it has been occupied by bandits who use it to raid the villages around. Even worse, the famous chapel in the castle has been desecrated and used for dark rites of some sort. At least that's what the priests of the town have been told in their dreams for two weeks now. And now something started killing the peasants' cattle. Someone has to go up there and stop this madness! Unfortunately the castle is known to be impenetrable. Nobody ever managed to capture it. So far...