Walkthrough:Kingdom Hearts Birth by Sleep/Aqua/Castle of Dreams
Upon entering this castle, Aqua will be awed by the inside of it. Suddenly, a woman passes by her, running away from the Grand Duke. Terra will also be present, where he and Aqua will converse on beleaving in dreams, while Aqua will mention that Ventus ran away from home. After the cutscene, walk up the stairs and enter the Ballroom.
A cutscene begins. A woman and her two daughters walk pass Aqua, who then senses a frightening amount of Darkness in their Hearts. She asks the Grand Duke on who they are, and he tells her that the woman is Lady Tremaine and the girls are her daughters. Aqua senses that there is something wrong with them. Prince Charming then appears, having mistaken Aqua for the girl he danced with, but then relises his mistake. The Grand Duke tells the prince that he's found a glass slipper and that he's going to search the entire kingdom to look for the girl who's foot fits the slipper, starting with Lady Tremaine's mannor. Aqua decides to follow him, in the hopes of sending three evil women to their deaths...
Exit the castle, and then reach the mannor, where a cutscene begins. Lady Tremaine is introducing her daughters, Anastasia and Drizella, to the Grand Duke. Aqua then grabs her Keyblade and heads towards the door, hoping on killing the evil trio before their Darkness harms anyone. However, a Fairy Godmother appears and tells her not to, as fighting the Darkness with Light is dangerous. The reason behind their darkness is because they are bitterly envious of Cinderella's beauty and Light. Shortly after her father mysteriously died, the Tremaines abused and mistreated Cindy, making her do all their chores while they lived like queens, with Lady Tremaine giving her very-selfish daughters everything they wanted.
One of Cindy's friends, Jaq, is trying to rescue her from being locked inside her tower, while Anastasia and Drizella try on the slipper so they can marry the Prince. Help Jaq by guarding him from the Unverse until he reaches the other side of the room.
Soon, Aqua sees the Grand Duke leaving the mannor, and suddenly returns to her normal size. Despite being known to not be the girl that the Prince is looking for, Aqua asks if the slipper would fit her foot, hoping to buy Cindy some time. Just then, Cindy appears, and tries on the slipper. However, Lady Tremaine trips over the Grand Duke, making him break the slipper. Cinderella then tells him that she has the other slipper, which fits. As they leave, the Tremaines' envy goes so high, that they decided to do the most evil thing they ever did...
Soon, it's revealed that a monster is attacking Cinderella. Lady Tremaine, Anastasia and Drizella have summoned it with their Darkness (now at just about 100%, making them nearly as evil as Vanitas and Maleficent), and are using it to murder Cinderella, but a bomb soons drops on the trio, either killing them, turning them into Heartless (most likely Darksides) or blasting them off. Aqua then fights off this beast, the Cursed Coach.
After defeating the boss, Cinderella and the Prince are reunited, completing the world's story.