My general impression is that, as a shorthand, in terms of technology and culture
the Aslene are like a combination of Vikings and Mongols from the Dark Ages;
the Alderlanders are like Saxons from the Dark Ages.
How common was slavery to these 2 cultures in the game outside of the Forbidden Lands?
I picture the Aslene most of the time like the Rohirim from Lord of the Rings
- but I think Vikings with horses instead of boats are a good despriction too.
My general impression of the Aslene is that they despise slavery. I think it is somewhere written that they do not even have fences, because they say that the land and animals belong to everyone. Nevertheless I think it ist also stated that they have a strict hierachy.
The Alderlanders look for me more like stereotypical/civilized knights. They have a lot of technology and built things like the Iron Locket.
I think the books hint that Alderland is very overcrowded so my guess is that the nobles pretty much abuse their feudal system.