Latin Triarii

Summary
Triarii are a powerful late-game brigade exclusive to the Latin faction group, with other faction groups not having a direct equivalent. They are very defensively oriented spearmen, similar to hoplites but lacking the ability to enter phalanx formation to further boost their defenses in pitched battles.

"Representing the oldest and wealthiest members of the legion, the Triarii wore the best armour available and fought like hoplites with a large shield and spear."

- In-game Description

History
After the Roman military reforms of the 4th Century BC, the Triarii were created as a third battleline behind the Principes and Hastati. They were the heaviest and most dependable infantry, continuing to fight in a fashion similar to the earlier hoplites. They would gradually move away from typical hoplite equipment and replace their round shields with the rectangular scutum shields also used by Hastati and Principes (as depicted in-game).

Tactics
At 500 defense, Triarii have the highest base defense of any infantry in the game.

However, Triarii lack access to the powerful phalanx formation which provides an additional +50% defenses, meaning that Triarii do not outmatch Heavy Hoplite brigades as long as the latter's phalanx formation holds, with Greek Heavy Hoplites and Hypaspists actually being markedly stronger. The true advantage of using Triarii over Heavy Hoplites is therefore not their higher stats but their greater flexibility, having similar stats to a Heavy Hoplite in phalanx formation, without the possibility of losing those stats if the formation breaks.

As such, the Triarii will outperform Heavy Hoplites in situations where it isn't possible to stay in phalanx formation, such as when outflanked or maintaining sieges.

The Triarii are arguably the best brigade in the game for sieging cities, and can also be very useful on the battlefield to complement the more flexible and offensive Latin Principes.