Laputa: Castle in the Sky (1986) Anime Watch

This high-flying adventure begins when Pazu, an engineer's apprentice, finds a young girl, Sheeta, floating down from the sky wearing a glowing pendant. Together they discover both are searching for a legendary floating castle, Laputa, and vow to unravel the mystery of the luminous crystal around Sheeta's neck. Their quest won't be easy, however. There are greedy air pirates, secret government agents and astounding obstacles to keep them from the truth, and from each other.