The Mobile Fortress Gradia is the fourth dungeon in Tales of Arise and can be found floating in the middle of the ocean after Almeidrea liquifies the majority of Mahag Saar's Dahnans. Unlike some of the game's other castles and fortresses, there's no way for players to return after completing it, meaning that many of the treasures onboard are missable if players don't fully explore the ship before tackling the boss.