The Juggernaught has more hit points and range and causes damage to a larger area than the Catapult Trireme. Like the Catapult Trireme, the Juggernaught is armed with a Catapult, which can fire at a location instead of at a particular unit. You must research Engineering before you can upgrade to the Juggernaught.
Researching Alchemy increases attack strength. Ballistics increases accuracy. Engineering increases range.