Slavery is a thing, but I wasn't sure whether or not it was economically important enough to merit an industry. Slavery is vital to the farming, mining and shipping industries, however. Here are the population requirements for each holding:
Farms: 25 slaves, 5 plebs, 1 noble
Mines: 30 slaves, 10 plebs, 1 noble
Ships: 10 slaves, 15 plebs, 2 nobles [galley slaves, sailors, captain and first mate]
I could conceivably add slavery as an industry that produces the 'Slaves' good, which is consumed by Farms, Mines and to a small degree Ships. I could set it up so that you can only produce Slave Pit holdings by either capturing slaves in war or starting with them as a holding. Right now (if implemented), Ragona would get all its slaves from outside sources.
If you guys think this is important enough an industry to model (which for the time, it might be), I'll add it in over the next couple of turns.
I've also been modelling wages this turn (and boy have prices skyrocketed to balance - I've had to do a lot of tweaking thus far to get the holding values near to what they are currently worth), but this would also technically permit a new kind of Holding if people were interested - Rents. This would be an industry based on the total population in Ragona, essentially taking wages from the common folk in exchange for a place to live. Might be trouble to set it up to recognise rent from slaves, plebs and nobles differently, though. I'll look at it at another date if folks are interested.