Lupin III: Princess of the Breeze - Kakusareta Kuuchuu Toshi

The story is set in Shahalta, a tiny European city-state floating in the air with practically no contact with the outside world for years. The government is transitioning from a monarchy to a republic, and to celebrate the occasion, a national treasure exhibition is being held. Lupin infiltrates the exhibition for the secret royal treasures, but not before sky pirates snatch them first.(Source: ANN)