In short: A secret meeting with Aelinore can end in two ways. To receive this quest, you first need to talk to princess in the garden after the meeting with the Duke. The quest is given to you by the maid Mirabelle, who is usually standing in front of the throne room.
Difficulty level: Easy. Before taking it, make sure you have the Skeleton Key (you can make it by combining Shackle with Iron Ore).
After taking the quest, wait until the evening (i.e. at the inn) and enter the castle area. After dark there'll be guards here, so you'll have to avoid them. First, go right into the garden and talk to Mirabelle.
To get to the tower, go up to the first floor of the castle. Use the side entrance (near the garden). Before you go in, make sure that the guard has moved away from the door.
Be cautious in the throne room as well (it's patrolled by one person). When the coast is clear, run up the stairs and make a turn for the tower.
The quest will proceed further only if it's the night - otherwise the tower will be empty.
If the time is right, there will be a cutscene. Unfortunately, the ruler will quickly appear on the scene and start... chocking Aelinore. You have a choice: hide behind the curtain and let the woman die or reveal yourself. It's worth doing, because this way you won't lose Aelinore and will be able to complete the next quest (a rescue mission).
No matter what you choose, you'll end up in prison. To get out, you can bribe the guard or use the Skeleton Key. After leaving the cell, use the secret passage hidden in the cell at the end of the room. The corridor leading to freedom is full with Snakes and Spiders.