Before you can reach the mysterious hills where the Marchenghast castle resides, you must open up Golden Hills Valley. You can find the steps on how to do that in this article: How to unlock Golden Hills Valley
But here is a little quick guide:
- Reach player level 12;
- Follow main story quests until you reach the quest “Why is the gate locked?”;
- Together with Councilman Gilbert and the Blacksmith Conrad, a key will be forged;
- Use the key to unlock the South Iron Gate!
Once inside, you should see a quest named "The Mystery of Marchenghast Castle", which is the one that will lead you into the mysterious area of Marchenghast.