Since there doesn't seem to be a thread for this yet, thought I'd start one where newcomers can ask their beginner questions. Ideally not walls of text but quick, focused answers and replies that are easy for others to check as well.
Btw linking this fabulous spreadsheet here, it has helped me out a ton already - thank you to 'YD' who created it! Really hoping we'll eventually get a 'Grognerd's guide part 2' type of polished masterpiece for Kingdoms
https://docs.google.com/spreadsheets/d/ ... id=0#gid=0
I'll start since there's sure a whole lot I'm still ignorant about

Castle buildings:
Are there any particular conditions for building these? Specifically I can't seem to trigger the 'Shell Keep', even though the region has 27 fort. points & 4 total mil buildings. Also the 1st and 2nd level permanent fortifications (3 & 10 points) were mutually exclusive. Is that equally true for the 4th level even though they are at different point amounts (26/28/30)?
Siege Workshop:
How can I recruit trebuchets etc., what are the other conditions besides having the building?
Unit Availability:
I had a stint playing as France, and what I really enjoyed were both light archers and peasants being very cost effective for what they provided (highest support value and outflanking/early siege capability respectively). However I've been sort of surprised to not have them available for other factions (Castilla/Bohemia etc.).
Not quite sure what my question is, but I guess basically how have people found ways to play successfully without those basic starting units, or how to get access to similar functioning units? For example playing as Castilla I felt completely lost without peasants and couldn't essentially siege even the least fortified settlements (Qasaba).
Court Expenses:
Does anyone have solid info on what affects it? In a playthrough my personal demesne size stayed basically the same, but this cost skyrocketed from ~ 50 in the start to over 300 gold.
Trading System:
What's the best approach for Kingdoms with the limited resource system? Coming from Empires, I'm accustomed to basically always satisfying a need with a resource within trading range. However already I can see that's not often possible here. For example with how the amount of tools/brass needed for certain buildings outstrips the ability to produce - or alternatively lacks vast amounts of the raw metal required. Is mostly compensating with high income commerce buildings the best strategy here?
Thank you in advance!