Eiyu Senki Gold: A New Conquest

The Far East kingdom of Zipang has kept itself sealed from the outside world under the rule of supreme hero Yamato Takeru, building something unique in isolation. But the hero of Oushuu, Masamune, is bored. When she finds a young man washed up on the shore with no memory of who he is, she sees an opportunity: put him in charge, challenge Takeru, and if he wins, Zipang breaks its isolation and begins a campaign of world conquest.