Complex sentence: He follows her down into a valley where her village rests (built around the ruins of an old city), where he starts to decipher their language, and discovers that traction cities are considered a fairytale.
Split into simpler sentences: He follows her down into a valley where her village rests in the ruins of an old city. He starts to decipher the language of the villagers, and discovers that traction cities are considered a fairytale.