The Singing Ice (System Neutral)

From publisher blurb: The party enters a castle that at first seems to be abandoned. But something is not right. It is empty and cold but clean and cared for. The princess has been turned into a beast. The spell will not be broken until a special crystal is placed in the highest tower. If that is done, it is proof that the princess learned to ask for help. The crystal is found on the other side of a portal, guarded by a dragon that should not be awoken. But it not just to walk up to the portal and get in. Where is it and how does it work? And will the team notice that they are about wake a dragon before it is too late? As in most of my adventures the focus is around the story and its characters, not the fighting. If fighting is of interest, it is possible to add enemies.