To access it, you'll need to:
1. Find the key: This is usually found hidden somewhere within the castle, often as a reward for completing a puzzle or task.
2. Unlock the door: Once you have the key, you can unlock the door leading to the restricted section, which is located near the main library area.
Keep in mind that the exact location of the key and the door may vary slightly depending on the version of the game and your progress.