Yeah, what Yolan said. Don't mean to discourage you, but you're going to lose a lot of the vision once it turns into a design document and even more when it turns into code. Realism needs a lot of research. Game mechanics to handle realism are even harder to merge, because there's a fine line between a realistic game and a boring simulation.
But IMHO, it's a little too complicated. You have a farm sim and a RTS there. You have combat - is it going to be RTS style or Mount & Blade? You have a city builder. And then a strategy aspect, with conquering other fiefs. That's a lot of games in one, and I worry it's going to be too much to do to enjoy.
I think it's a bit more fun if it's in some smaller pieces. If I was doing it, I'd split it into Fief I, II, and III.
Fief I has purely city building and farming aspects, no combat yet. Combat is abstracted out - if you buy 200 soldiers, they defend your city and all, but you don't see the fighting.
Fief II would add RTS features to the first game, but keep it at only defense mode. You'd be encouraged to build a castle, and to keep the farms outside of the castle (because of building costs or whatever). It'd be something like Stronghold, with no attacking, and a lot more realism.
Fief III would then expand the RTS feature to allow you to go out and capture other fiefs, and maybe even add an aspect with you serving the monarch and all. Add quests. etc.
That way you have manageable chunks for people to play and comment, and get a more realistic outlook of any possible problems with the design. You also get a lot more satisfaction, because you're completing some goals
Just my humble opinion