Automation with Peasants
If you are a king with no subjects, what are you king of?
Space Engineers and other crafting games have a progression. You start with your bare hands, use them to make bad tools, use those to make better tools, and so on. At a certain point you switch methods and automate the thing that gets boring, Space Engineers does this through small and large ship welders and grinders.
Medieval Engineers only has the tools, and in the end you will still be mining and building castles by hand. If we had small "camps" of NPC's that we could assimilate into our kingdom, we could use them to automate menial tasks as they get boring. We could give them roles such as Cook, Miner, Hunter, Soldier and Builder. The difficult part for the players would be the control of the peasants, and we would need to give them specific work areas. We would not want the hunters straying on enemy land or the cook cooking the wrong meals. Builders are simple enough with blueprints but players would still need to identify proper stockpiles so they dont pull from the wrong buildsite.
The more I think about this the closer this gets to Dwarf Fortress.
At least now you are present to be properly knelt to.