Okay so i've made a few suggestions with reguards to buildings but i thought i'd make more . an hope they might not be to hard to add.

Slave market - would add a permanent ransome broker to the court yard of the buildable castle(s). so you could buy and sell slaves permanently withouth aving to track down a slaver.

Outlying farms. - Would generate assorted food items in the castles inventory and further reduce the maintenence cost of garrisoned troops, as well as add slight boost to income through castle tax.

Inn - Works like the Tavern , with mercenaries, and other travelling characters to spawn at the castle.

Recruitment post - would allow generation of units based on current income -or allocated funds- so each week depending on the state of the castle would add 1-10 soldiers to the castles garrison each week. of recruit level.

training yard - i mentioned this in another thread. would upgrade soldiers in the castles garrison potentially based on the same funding as the recruitment pool. would also permit you to increase a specific units stats.

Officers quarters: would allow the recruitment of a Guard Captain. which is common in a lot of mods allowing you to create patrols. and allocate funds for the castles military to use. pre-requisite for building the recruitment post and the training yard.

Blacksmith - would reduce the recruitment cost of customizable player faction units. as well as add a weapons merchant with which to buy weapons. could have multiple upgrade levels with each level offering better equipment.

Armour smith - similar to blacksmith but with armour. upgrades, armour quality , would further lower the unit recruitment cost.

the blacksmith an armour smith would allow the arguement that the cost of the units is so high because the castle has to import the equipment so building these would reduce the cost. perhaps each level would lower the cost for its tier of unit. so the initial black smith would only reduce the cost for recruits, and militia.

they could also have items specific for upgrading the units that would tell you how much the upgrade would cost. this would in turn make it a little clearer an easier for people to upgrade.

Merchants guild chapter - would allow you to build an enterprise in the castle by adding a guild master to the castle court yard. along with a slight boost in income from additional trade taxes .

Knights chapter - would generate units of Knights to patrol around the castle. one knight and a few squires.

Militia Quarters - would be similar to the Knights Chapter but would only produce recruits, and Militia to patrol for a small upkeep cost. - could allow an RP explaination as to how troops are lvling up in the garrison. as well as how recruits are being attracted.

building the militia quarters, would allow the area to be safer so could also generate 'approval' from the castles residence improving your relations wit hit.

perhaps some buildings could also reduce your relations but give you decent bonuses.

for example maybe you could build a bandit refuge which would generate hostile parties in your area that would fight with your knights, and militias. and of course something for you to fight during peace time. but would gradually lower your castles relations with you.

Religious buildings - i'm not sure how this would work with Atheism since its not a religion but something you could build to increase religious unity. perhaps something like

'community centers' as a generic term which would help increase the growth of the castle an thus the income. but would also increase religious unity. the actual religions could of course use their own cultural equivalent for their religion - mosque, synagogue - temple - etc.

these are just some basic ideas.