Hmm, a lot of these ideas would take a looooooooong time to put in, so I've got a more dumbed own version of a castle.
A. The castle would have a stable, where you can buy and sell horses, but all of these horses are cheaper for you, but ones you sell sell for the same as you would to a normal merchant.
B. A barracks which generates five soldiers a week (they start off as peasants of your faction, if you have no faction, then they start off as peasants and progress as the zendar peasants do), and the longer they stay in the barracks, the higher they advance, you can also place units in the barracks to have them be trained higher. Once the units gain enough experience to gain a level, you have to go back to the barracks and level them up to whatever unit you want. Units in the barracks are the ones who defend the castle as the garrison. The speed at which units are trained in the barracks is your own training level but with an extra 25%.
C. An armory, which is basically the armor and weapons merchants, but all of the stuff is cheaper.
D. A storage area with a lot of space.
E. A merchant who sells you food and horses that may not be in the stables.
There where a couple of other suggestions I had, but those are not as important and i forgot them right now, i may add them later.