To build castles in the air means to daydream, and in this sentence she is daydreaming about the two of them and what they could mean to each other.
Context: "He couldn't let her fantasize about the two of them and what they could mean to each other."
Hypothesis: He couldn't let her build castles in the air about the two of them and what they could mean to each other.
Based on the context, is the hypothesis true, false, or unknown?