My idea was to make a castle interior possible to enter without loading screens, similar to the gate interior in sieges.
But I might try do a interior for the castle in blender, maybe the interior could be rescaled.
An interesting thought. Some of the smaller buildings in the scene editor (some tents and shacks) have a door and can be entered. In addition, many of the castle gate houses and towers and the like can be entered. However, in terms of interior rooms and halls and the like where you would want to add detail and character, these are typically designed as their own scene.
I'd imagine the biggest reason is that this allows interiors to be customized via the scene editor rather than constrained by whatever their outer proportions are for the exterior asset. Imagine, for example, having to redesign an entire town space because you needed to make certain rooms, etc. bigger after the fact. Or having to deal with looking at 100's of objects inside something else you added. It would make the viewing rather tedious. It is weird if you are used to building in something like say, the sims, where interiors and exteriors are handled together. However, handling these separately has its benefits when you want to design complex interiors.
So, in terms of the mod tools, those two files (I think that is what you saw?) were designed with this intention and are two completely different scenes. There is no way I can think of to try and merge the two together. If you want them to exist in one setting, then both exterior and interior need to be designed in one scene. This is how it is done with any interior for MP for example. However, those interiors are somewhat limited in their complexity from prefab. It doesn't mean you can't improve upon them. For example, below I have made a prefab gatehouse in the scene. If I go into it, I can still add stuff to accent it. Through the door in second image you can see an unfinished part of the rest of my scene.