The Mob is a militia unit that can only be created using the City's Call to Arms ability or the Populist's Recruit ability. They may be completely unarmoured and wield only a dagger, but these angry citizens will fight hard to defend their homes and if they can't overpower the enemy with sheer numbers, at least they will buy the player some time to bring real soldiers in. However, as time goes on the fervour of the mob dies down, this being shown in game by their health gradually descreasing. They won't be automatically killed, but will be left at such a low health level that one attack from any unit is enough to finish them off.
The number of Mob units spawned when activating the Call to Arms ability at a City can be increased by constructing Peasant Dwellings and a Sporting Centre there and by increasing the city level (Peasant Dwellings is the new name for the Small House building).
Geography and a general lack of resources would mould Greece's destiny, creating a society where pride of place and a strong individual identity based on one's community or polis would take precendence over that of the individual. For this reason, the Greeks were highly territorial, fighting aggressively to defend their homes whenever so threatened by outsiders, even when facing suicidal odds. This martial tendency manifested itself as late as the Second World War, where German paratroopers on Crete who landed in civilian areas faced extremely ferocious resistance from the local population.
Despite being a Roman faction, the Roman Republic in Imperivm Invictvm can spawn Mobs to attack you, so beware. They may not cause much damage, but they can certainly distract you, especially if you seize Rome before you are prepared to face them.