One Piece: Mega Mecha Soldier of Karakuri Castle

The crew salvages a treasure chest from a sinking wreck, but inside turns out to be an old lady hiding. To get the Straw Hat Pirates to take her home, she promises them the treasure of a golden crown on her island, Mecha Island. When they arrive, their ship is first attacked by the lord of the island. But later he decides to use the pirates to help him solve the riddle of the Golden Crown.
(Source: Anime News Network)