well... interesting question... my original plan was to just make the outside of the castle modules without making rooms inside. In order to match the Outerra world scale these models are already pretty big, and I figure most people would probably just use them as background "fly-by" scenery. However, I probably should at least add collision so characters can walk along the walls, stand in the archer turrets and on balconies etc.
For my purposes, I make a second version with collision that is used in my animation software, and when I need to have character interacting with the models I do it as close-ups in that environment. I just use the Outerra models to give the scenes a sense of scale in the real world. Any people in those scenes would be green screened onto a different layer.
If you downloaded that last batch of stuff I put on my dropbox, there was a log cabin that had an inside with furniture that can be seen through the windows. But until recently I didn't see much evidence that people were using characters in their world, so I didn't put collision so people could walk up the stairs, stand on the porch and go inside. Maybe I should go back and add collision to that cabin.
There were also 2 stone bridges in the campground models, and they both have collision. If you put them across a stream, cars can drive across and small boats can float through the opening under the bridges. Using the models in that set (pavilions, tents, picnic tables, campers, park signs, bridges, cabins etc) you can make a pretty cool looking park in the mountains.