A bireme is an ancient oared warship (galley) with two decks of oars, invented and used by Greeks even before the 6th century BC. Biremes were long vessels built for military purposes and could achieve relatively high speed. A bireme is an ancient oared warship (galley) with two decks of oars, invented and used by Greeks even before the 6th century BC. Biremes were long vessels built for military purposes and could achieve relatively high speed.