Tremulous is a game dedicated to multiplayer, so if you want to play single player in this game, you will be disappointed. However, it should not be hard to find a handful of friends in order to organize a round of more in this nice three dimensional game.
The game allows two teams to play at the same time, one being the human team while the other is the alien team. Each of them has unique weapons, stats, advantages and disadvantages so you should choose wisely who do you want to play with in order to have the better position over the arena.
