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.