Internal 3x3. <BR>
2 storys. Build with walls (walls have inhernet floor on top of them)<BR>
Put a stair in one corner.<BR>
Place floor on the 7 squares inside the walls, but not the center square  or over the walls themsevles.
Build the 2nd level walls, corners first, but instead of one center wall put in a stair.<BR>
Move to the roof and build a floor over the 9 inner squares.<BR>
Deconstruct the stair (both parts) leading to the roof. Replace with a wall.<BR>
Put a fortification dead center  of the 2nd floor.<BR>
Carve the 2nd floor into fortifications.