Here is a basic list of what will most certainly be inside the castle:
-Entrance (lobby)
-West wing (a room dedicated to room entrances)
-East wing (see west wing)
-A few empty rooms where Chancellors, Advisors etc can be housed
-Small armory for the Sentry/Guards
-War/ready room (basically a room with tables and chairs)
-A dining hall (also a bar.. it will be similar to the 2k1 castle
-Throne room (the life of the party

)
-Guest rooms where guests may be housed
That's all I can think of for now. More will come, I'm sure.
Please keep in mind the following: As far as I know, the castle is to be underground. The only entrance into the castle I know of is dungeons stairs, unless something else is created, so please don't tell me to make watchtowers and outlooks because I can't do that. I will ask Stefan if there is any other way to get a castle entrance that would be nice later on. The entrance will be the last thing I do, anyways.
So let's hear some suggestions