The Trireme is one of the first warships available in Civilization IV. It is used primarily to attack or defend Galleys. It can be upgraded to a Caravel if Optics is researched, or a Frigate if Chemistry is researched.
Triremes are especially effective in combat against Galleys, but they have no transport capacity. They cannot enter ocean spaces.
Triremes were ancient war galleys with three rows of oars on each side. The trireme was an improvement upon an older type of ship called the "pentekonter," which was similar in design but possessed only one row of oars on each side. (The standard "galleys" found in Civilization IV are pentekonters.) The staggered seating of the rowers on the trireme allowed for more people to be packed into roughly the same amount of space, and (more importantly) the extra manpower allowed triremes to travel at faster speeds. Some ancient sources claim that the trireme existed as far back as the eighth century BC, but the first known account of their use was by Polycrates of Samos in 525 BC during an invasion of Egypt. By the middle of the fifth century BC, triremes were in common use throughout Greece.
The average trireme was about 35 meters long, carrying 170 oarsmen, 20 crew members, and 10 marines under the command of a captain. According to ancient sources, triremes could reach a maximum speed of about 20 km/hour in battle, or sustain a slower speed of 13 km/hour for a long stretch of time. Triremes also possessed sails but they were not used during combat. A Trireme's main offensive weapon was its "beak," a stout piece of sharpened wood (often clad in metal) which protruded directly forward from its bow, at or below the waterline. In battle the primary tactic was to use the beaks to ram and sink enemy ships, which required building up as much speed as possible to punch through the opposing ship's hull. This was a difficult task and required a great deal of cooperation from the rowers, as even a single rower moving out of place could greatly slow the ship. For this reason the rowers on Greek triremes were not slaves but instead well-trained free men who received good pay for their services.
During the Persian War, the Greeks used their triremes to defeat the Persians at the decisive Battle of Salamis. Triremes also formed the backbone of the Athenian navy during the Peloponnesian War. As the war progressed Sparta and her allies created their own powerful fleet of triremes; these would be put into good use in 404 BC during the Battle at Aegospotami, when Lysander destroyed Athens' navy, cutting off the city-state's trade and eventually starving her into submission.
Triremes would eventually be supplanted by the larger and faster quinqueremes, but the design was revived years later by the Romans, who, facing no major naval enemies, preferred the smaller and cheaper triremes to the more expensive quinqueremes.