The plot opens on the royal family of Paflagonia eating breakfast together: King Valoroso, his wife, the Queen, and their daughter, Princess Angelica. Through the course of the meal it is discovered that Prince Bulbo, heir to the neighbouring kingdom of Crim Tartary, and son of King Padella is coming to visit Paflagonia. It is also discovered, after the two females have left the table, that King Valoroso stole his crown, and all his wealth, from his nephew, Prince Giglio, when the prince was an infant.
Prince Giglio and Princess Angelica have been brought up together very closely, Princess Angelica being considered the most beautiful and wisest girl in the kingdom and Giglio being much overlooked in the household. Giglio, besotted with his cousin, has given her a ring belonging to his mother, which, unknown to them, was given to her by the Fairy Blackstick and which held the power to make the wearer beautiful to everyone who beheld them. After an argument with Giglio, about the arrival of the long-awaited Prince Bulbo, Angelica throws the ring out of the window and can be seen for her own, less attractive self.
Prince Bulbo, in his turn, possesses a magic rose, with the same power as the ring and coming from the same source: the Fairy Blackstick. Upon his arrival, this causes Angelica to be madly in love with him.
Angelica's governess, Countess Gruffanuff, finds the magic ring in the garden and, whilst wearing it, convinces Giglio to sign a paper swearing to marry her. She then gives the ring to Angelica's maid, Betsinda, an orphan discovered by the family with a torn cloak in her possession. The maid, however, is actually Rosalba, the only child of the true king of Crim Tartary. When Betsinda wears the ring to take the warming pan around the bedrooms, Princes Bulbo and Giglio immediately fall in love with her, along with King Valoroso. This excites the rage of The Queen, Angelica and Gruffanuff, and causes Betsinda to be driven from the house.
In response to Giglio's rudeness, Valoroso orders him to be executed, but his Captain of the Guards, Count Kutasoff Hedzoff, takes Bulbo to the scaffold instead, where he is reprieved at the last moment by Angelica, who takes his rose, returns to her former beauty and marries him.
Giglio is forced to flee and, with some help from The Fairy Blackstick, disguises himself as a student. In the meantime Rosalba has returned to Crim Tartary and discovered her heritage by means of the torn cloak, which is reunited with its other half to make the words "Princess Rosalba". King Padella, after his offer of marriage is refused, orders Rosalba to be thrown to the lions. Giglio, upon hearing this, takes back his throne in Paflagonia and leads his army to rescue Rosalba, using the captured Bulbo as a hostage.
When Padella refuses the exchange, Giglio decides that he had better keep his word and put Bulbo to death as threatened. However, the lions set upon Rosalba happen to be exactly the same lions which she grew up with in the wild, prior to being found by Princess Angelica, and carry her on their backs to Giglio's camp, where the pair are reunited.
Giglio and Rosalba return to Paflagonia along with Bulbo, now wearing the fairy ring. When they sit down to breakfast on their wedding day, Gruffanuff produces the paper pledging Giglio to herself. Wishing to put him in his place for his earlier arrogance, the Fairy Blackstick refuses to help at first and Giglio is forced to take Gruffanuff to the church in Rosalba's place. However, when they reach the building, The Fairy Blackstick transforms the doorknocker back into Gruffanuff's real husband, long believed dead after being bewitched by the fairy herself many years before. Giglio and Rosalba are then free to marry and do so. The Fairy Blackstick then leaves, never to be heard of again.

Question: Why did King Padella sentence Rosalba to death?
Answer: she refused his marriage proposal
Alternative answer: For refusing to marry him.