Something you'll start to see in the next update:
Freedmen is a great way to increase the population of your cities. There are 3 levels with I, II, and III. Each level increases food production by +5 in the city.
However, there is a price. The disease of slavery and finally freeing them reduces the loyalty and quality of your armies, and eats away at your empire.
Freedmen I discovery causes War Deity to become obsolete.
Freedmen II discovery causes Barracks and Tax Collection to become obsolete.
Freedmen III discovery causes Mint, Happiness Deity, and many other Happiness structures to become obsolete.
Additionally, each Freedmen "structure" reduces the defensive effectiveness of the capital palace and city walls by 5% in all your cities.
So you can see here where Slavery evolves into a detriment for the empire.