Palace of Mirrors by Margaret Peterson Haddix
Cecilia is not really a commoner. She is the true princess of the kingdom, but she is hiding from the enemies that killed her parents. One day, she decides to go to the palace, along with her best friend Harper, and save the girl, Desmia, who is pretending to be the princess from danger. When they gets to the palace, Desmia locks them in the tower and leaves them there. Will Cecilia and Harper be able to convince Desmia that Cecilia is the real princess, or is Desmia working for the people who killed Cecilia's parents?
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