Lupin III: Princess of the Breeze - Kakusareta Kuuchuu Toshi

ルパン三世 princess of the breeze ～隠された空中都市～

In Shahalta, a tiny European city-state floating in the sky has had no contact with the outside world for a number of years. The government has went from a monarchy to a republic, and to celebrate the occasion, a national treasure exhibition is being held. Lupin decides to infiltrate the exhibition to steal the secret royal treasures, only to find out that a gang of air pirates snatch the treasure first.

(Source: Anime News Network)