I would still create it in blender, but in this case you can also do it with the BSP brush -> but after you are finished, convert them to a mesh (better performance) + probably you will have to export it into a 3d program to create the lightmap.
You could do that with LOD’s ->
LOD0 = the entire building will be displayed
LOD1 = just the entrance and the outside of the castle
LOD2 = just the outside of the castle