I read this book many many years ago. It was about this castle with these doors that lead to other worlds. To open these doors the guy had to go through these trials to earn the key to the door and or world. He had to find all the keys before some evil got out and took over. The last door was in the castle tower.

I believe there was only three main characters, the king and or master of the castle, the librarian and the time keeper. The time keeper plays a big part towards the end of the book.

I believe I remember the cover having a drawing of an old wooden door.