Flagship

Summary
The Flagship is the most advanced and most expensive warship in the game, capable of bombardment at long range. It is available to all faction groups except the Gauls and Illyrians.

"Ships with more than five banks of rowers were relatively rare and often used as command ships in ancient navies. Requiring large crews to operate, these flag ships were large enough to carry artillery on deck and almost impossible to sink by ramming."

- In-game Description

History
After the invention of the quinquereme around 400 BC, with 5 files of oarsmen on each side of the ship, it didn't take long for even larger polyremes to be constructed. Hexaremes (with 6 files of oarsmen) were invented in Syrakousai in the early-mid 4th Century BC. Septiremes (with 7 files) were likely first commissioned by Alexander the Great of Macedon in the 330s BC, while the octeres (8 files), enneres (9) and deceres (10) all made their first appearances in the late 4th Century BC as well.

They could require many hundreds of rowers, sometimes even more than a thousand, and could additionally carry hundreds of combatants and several ballista catapults. These ships were often used as flagships or command vessels, carrying the fleet's commanding officers.

These larger warships were notably more common in the eastern Mediterranean, in the navies of Macedon, the Seleucid Empire and Ptolemaic Egypt, than they were further west. Though the deceres seems to be the largest ship type used for battle, the Macedonians and Egyptians are known to have experimented with even larger vessels. These huge ships were probably too unwieldy to be used in practice, and may have only been built for prestige purposes. The largest known type is a 130-meter long tessarakonteres (40), constructed in Egypt in the 3rd Century BC that required no less than 4000 oarsmen.

Note that the number in the ship's name did not refer to the amount of oars, but rather the amount of rowers. For practical reasons, it is likely that most or all of these ships used 2 or 3 rows of oars similar to biremes and triremes, but with long and powerful oars that each required several oarsmen. Hexaremes, for example, likely had 3 rows of oars per side just like a trireme, but with each oar being manned by 2 oarsmen for a total of 6 files of rowers per side.

Tactics
Flagships require a further progression in the naval skill tree than any other ship type in the game, aside from Large Transport Ships.

Despite being much more expensive, flagships are actually identical in melee combat (ramming or boarding) to Heavy Warships. Instead, flagships have a drastically increased missile range and better missile damage. Flagships are therefore best used to provide powerful missile support from afar, as it is more cost-effective to use the cheaper heavy warships for ramming and boarding.

The very long range of flagships also makes them viable for bombarding enemy land units that are near to the shore, or for attacking coastal cities.