Im no expert on the subject, but what I did to make my new scene (not a castle, but if I built a castle on the spot it would be

) is to first go into the game and use the land generater to generate a plot of land that I liked, then copy the terrain key. Then add a scene that uses that key to your module. With python you just open up module_scenes.py and add in another entry, using your key when you get to that part.
Then you just need to add a way to get to that scene in game, like making the castle party and having it trigger a menu (have to edit simple_triggers for that). Then have that menu point to your scene. Once your in, you can build your scene, and when you exit edit mode after adding stuff, just select save, and it will create a new file under sceneobj folder of your mod named after your scene with all the objects in it.
Oh hey I just noticed you said indoor scene

You still have to go into the scenes file and make a new scene, but then you need to make a body for the scene. I havent tried doing that yet though so I cant help you there.