Kumo no Mukou, Yakusoku no Basho, Tenkuu no Shiro Laputa Recommendations
Both are sci-fi and romance. Contains atmosphere of beauty and both have theme plot in the sky . Both have high-flying adventure. In both Main male character is an engineer. In both anime they vow to unravel the mystery. In Tenkuu no Shiro Laputa it is of the luminous crystal around Sheeta's neck and in Kumo no Mukou, Yakusoku it is about the tower. There quest won't be easy.