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Frozen: Why Anna Had To Stay In The Castle Along With Elsa
The beginning of Frozen explains that Elsa and Anna are kept in the castle, which makes sense for the former due to her powers, but it's unclear why Anna was also made to stay indoors. As children, the two sisters were very close, and Elsa frequently used her powers (which she could have inherited from Queen Iduna,) to make fun activities in their playtime. In one instance, Elsa is making ice platforms for Anna to jump onto, but Anna keeps getting faster, forcing Elsa to try and keep up with her. Under this stress, Elsa accidentally injures Anna with her powers. Their parents, the King and Queen of Arendelle, take Anna to the magical trolls who heal her but also alter her memories so that she has no knowledge of Elsa's powers. This fright led the King and Queen to isolate both Elsa and Anna from the kingdom.