Special effects

It looked to me that many of the battle scenes were actually double exposures. They filmed one take with all the explosions, and then they filmed another take with the troops marching across the battlefield. I can only guess that this was done so as not to endanger the actors. After this movie and before the modern age of special effects this kind of thing was a very complex stunt where everything was rehearsed beforehand and the actors knew not to step in danger zones.

In the movie in many of these battle scenes people appeared as "ghosts". There was one scene where a building was blown up as troops marched close by, yet none of the debris hit any of the actors.