The Legionnaire is the basic unit of the Roman army (can be built without research). He is widely considered a generalist and is the only unit in the game good to use for all three main functions (basic light infantry attacker/infantry raider/cavalry defender). In early game, legionnaire can be used as both raiders and attackers until being replaced by other specialized units (Imperian for attack and Equites Imperatoris for raiding). After that, he can be used as the cavalry defender.
To compensate this flexibility, his training is expensive compared to the Teutonic and Gallic beginning units, but has a significantly higher defense than the Teutonic Clubswinger and better attack than the Phalanx . He is good at defending against cavalry, especially when combined with praetorians.
Source: http://travian.wikia.com/ .