I made a model in Blender and when I imported it into Unity the walls were only visible from the outside. All the walls when looked at from the interior, disappear, but reappear when looking at the exterior side of them. Is this a problem with the blender model, or perhaps a Unity import setting?
If you have a shader that isn’t double sided (so most shaders, because of efficiency) Unity will only render the front side of the polygons. You can easily solve this by duplicating the building and reversing the normals of the copy. Ideally you should create actual walls with thickness though.