Q: It seems like the feudal state did not necessarily oppress its immediate threats, serfs. Rather, continue its oppression on the slaves like their ancient counterpart. This is rather different from what @VladimirLenin1918 wrote.
A: The oppressed class in feudal state is not necessarily serfs. They could be anyone that is being oppressed by the state apparatus, call it slaves or someone who are label as worthless or inferior. It is still undeniable that those class antagonisms exist albeit taking into another form.