Just give it modability, similar to how Minecraft doesn't support mods but gets hacked wide open and modded to hell and back. Do that, then come back with a DF overhaul that gives all your little yellows unique names and needs.
A big problem of these games, particularly originating with Minecraft, is that space is voluntary. In MC, you can successfully live in a 3x3 hole in the dirt. In DF, if you attempt this, you get your dwarves slaughtered and tantrum spirals in spades. This is because MC demands nothing of you, while DF demands everything of every dwarf. If Castle Story doesn't demand anything of the citizens, then I don't see big construction being fun. Sure, it's entertaining to build a classic castle, but if your workers just stand in the courtyard because they have no needs, then it gets boring awfully fast.
DF construction beauty is mandated, you must build something if you want to survive, and even the most bare-bones construction will be more than nothing. MC construction is about 98% voluntary, and you only see people building for fun. CS will need to demand construction of you, otherwise people will do one interesting project and leave, because there's nothing else to do. For this reason I really REALLY hope that CS workers will need some basic requirements, like food and maybe water, though I could see water being an FPS black-hole once you get a lake spilling over into the abyss. If certain actions need room, like how DF squads need barracks, then that'd add some space too. Really, anything that forces the user to create space is helpful, so long as you let the user define that space. The 3x3 barracks ala Age of Empires aren't fun. The Terraria houses of custom size, shape, light source, and hard surface, are much more entertaining.
Require the player to build something to advance, but let the player define the shape of it. Things will escalate from there and architecture will happen naturally.