Castle's have a lot of wasted space inside the first walls, Also they have the highest amount of lockdowns/Secures but with the narrow corridors and small rooms it makes filling them effectively almost impossible.
My Idea Is to Purchase a "Drill Yard Deed" For castles for 1,000,000 Gold making the small band within the walls placeable and lockable. Meaning it could be used for storage or in my case a place to add training dummies ect to make it appear to be a functioning castle.
Another idea i had for the Castle was the "stable Deed" which did NOT add an npc instead what it does is section off the bottom left corner of the inner castle with wooden fence and doors on either side with a spawner that adds 4 of like animals of your choice.
I.E A spawner that adds 4 Sheep or 4 Goats or 4 Horses ect nothing powerful only base animals that serve a purpose minus pack animals.
So the castle would have a stock of whool or meat or Horses for their cavalry units
Incase many of you havent see me in game i am a very Vanilla player i run around with a straw hat and a spear but i also am a very functional player. I like to make the most of the games basic features, i make my own cloth and bandages, mine my own ore for weapons and armor. when hunting i often carve everything off the things i kill, meat, leather and scales.