There are a few different units in CivCity: Rome.
Legionnaire (Legionary)[]
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Legionnaires with a Centurion
A heavy-infantry unit that is a little slow and has slow attack speed but has high defense and attack. They are melee fighters that only attack on the ground. Swords are required to make these units. They require Swords in your warehouse to actually spawn.
Historically, created after the Marian Reforms (changes to the Roman Army), Legionaries created the bulk of the Roman Army in the early stages of Rome itself. Legionaries were a kind of Heavy-Infantry, carrying a large rectangular shield and a short-sword known as a Gladius. Though they carried up to 2 Javelins, they don't appear in the game and are not used. A Legionnaire is forced to serve in the Roman Army for 25 years. Legionaries were used to be a kind of Police Force, protecting critical buildings and certain people.
Velites[]
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Velites (light gray uniform) with a Centurion
A light infantry unit with the lightest armor, and fast speed. Velites carry a large assortment of Javelins to use in combat against enemies from a distance. They are very weak in a close-distance fight with the enemy. They require Javelins in your warehouse to actually spawn.
Historically, Velites were a class of light infantry in the Roman Army of the Mid-Republic, they were armed with long Javelins to fling at the enemy, and a short swords known as Gladii's, which were variants of the Gladius which Legionnaires used. Gladii are not used in CivCity: Rome for the Velites. Velites were placed in the front during combat, and acted as a scouting, screening, and a skirmish force to ward off enemy attacks. Velites did not form their own units, instead they were in small groups leaded by either a Triarii, Hastati, or a Principes. Do note that Triariis, Hastatis, and Principes are not present in the game itself. Velites also had a distinct wolf hood over their head, though it is not featured in the game.
Centurion[]
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A Centurion
An Officer unit which commands a Century, which is a grouping of units up to 30 men. Centurions are used to lead certain centuries, you can have a maximum of 3, each commanding their own respective century. They fight in combat, and have similar statistics to a standard Legionnaire.
Historically, Centurions commanded a Century, containing up to 30-60 men. A Centurion is equivalent to an NCO (Non-Commissioned Officer) in modern day. Centurions were regular military units who rose up in the ranks of their Cohort, and they could also be elected by the Senate or State to be in command of their Century.