Historically the Hastati were the somewhat younger men in the Roman army. In the Rome - Total War setting, they belong to the light infantry class, they throw their pila first before charging the enemy. Overall the Hastati are a flexible unit in the game, however they can easily be routed by devastating cavalry charges or get destroyed by the heavy infantry.
Speculations were originally made whether or not the Hastati units were swordsmen or spearman
- Article on the historical Hastati on Wikipedia