The Good Magician's Castle is the residence and office for the Good Magician Humfrey.
The castle is located on the same site as the original Castle Zombie was during Roogna's life time. In order to reduce the number of visitors seeking answers, Humfrey frequently alters the paths that people use to reach his castle, preventing most people from knowing it's exact location. 
The castle was designed and built by centaurs as part of their year's service for help Humfrey provided.  Over 300 centaurs each owed Humfrey in exchange for his help in getting rid of the lightening bug problem that Trent had created to demonstrate his power to the centaurs. 
When attempting to gain entrance for the purpose of asking a question, one must overcome three challenges. The challenges are set up so that only those that are serious about their intentions will be able to get in and have they're question answered. Then the Good Magician Humfrey answers they're question in exchange for one year working or equal service. Some citizens end up serving their year by being part of a challenge for future visitors. They are usually treated quite well and some even enjoy their time at the castle.