This game, Zombie Army Trilogy, is a game with feature where you get more enemies the more players you have (or friends, for that matter, ofc higher difficulty also means tougher enemies and increased spawn/more numbers of enemies). Most characters would be during the WWI or WWII (I can't remember), the bonus characters are L4D1 and L4D2 characters where they have 0 voice lines in this game. As for weaps, find it out yourself which one suits you more. I would definitely recommend it since it is a good game to pop the zombies (albeit your enemies are Nazi zombies, you would need to fight the final boss - Hitler in the end but that's another story).