These units are from the Iron age. A Legionnaire has the highest defense among the Iron age units, but has a moderate attack and slow speed. They have a defense bonus on plain terrain and a combat bonus against light infantry, like other Heavy Infantry units.
These units are effective against both melee classes and cavalry classes, but are vulnerable to ranged classes, as both have combat bonus against Legionnaires. In addition to their slow speed, they are usually killed without attacking any unit when encountered with several ranged units.
It is recommended to combine Legionnaires with Cavalry to get rid of ranged units or with ranged units to shoot back.
They could also be used effectively against Mercenary units due to the combat bonus.
A little bit of History
For much of Ancient rome's history, the Legion was their basic and best unit. It could form massive formations, but were still mobile enough to outmaneuver the enemy. However, due to its heavyness, ranged units could pick it off easily.
The Roman legionary
The Roman legionary was a professional soldier of the Roman army after the Marian reforms. Legionaries had to be Roman citizens under the age of 45. They enlisted in a legion for twenty-five years of service, a change from the early practice of enlisting only for a campaign. The last five years were on veteran lighter duties.
On the march
On the march in unfriendly terrain, the legionary would be loaded down with armour, shield (scutum), helmet (galea), two javelins (one heavy pilum and one light verutum), a short sword (gladius), a dagger (pugio), a pair of heavy sandals (caligae), a sarcina (marching pack), about fourteen days' worth of food, a waterskin (bladder for water), cooking equipment, two stakes (sudes murale) for the construction of palisades, and a shovel or wicker basket.
The Roman soldier underwent especially rigorous training; discipline was the base of the army's success, and the soldiers were relentlessly and constantly trained with weapons and especially with drill — forced marches with full load and in tight formation were frequent. Discipline was important and infractions were heavily punished by the centurions. However, honours, rewards and promotions were frequently awarded to legionaries who distinguished themselves in battle or through exemplary service.