During various battles the archers are told to "fire" or "hold fire" also the artillery was told to "fire". The term "fire" didn't come around until the advent of gunpowder powered weapons. The correct command would have been "Loose".
Since this film is fantasy rather than history, some minor anachronisms are allowed, such as a Greek army using Roman tools or flags. Since many iconic "Greek myth" images come in fact from the Italian Renaissance, anything from up until the 17th century AD is fair game.