The Romans were very fond of fire-based weaponry. In fact, it's said they invented the precursor of the modern-day flamethrower. Slaves were fed huge amounts of beans and other flatulence producing foods and then forced to kneel with their bare bottoms pointed toward the enemy while their farts were ignited by torch-bearing roman soldiers. They were mainly used in short-range cheek-to-cheek combat. Though not extremely effective in terms of actual casualties produced, the enemies of Rome would often break into full retreat at the sight of hundred of hairy barbarians with blue flames shooting out of their asses.