A city must have iron in its Strategic Resource box to build a legionary.
The Roman Legions, created during the early wars of the Republic and tested during the three Punic Wars against Carthage, were the fighting force that conquered and held the Roman Empire together for five centuries. Each legion was composed of 6000 men, divided into 60 smaller groups known as centuries. Nearly unstoppable on the attack, the legion was relatively weak on defense. The legion's demise as a military unit was accelerated by the destruction of Emperor Valens' army by gothic knights in the fourth century.