For B) Step by step:
1. Watchtower Kit is deployed
2. After that deploy a fences-kit next to it (close position)
3. Put down trees to the watchtower and to the fences
4. Build both up with an shovel
5. Now add planks and nails to the watchtower
6. Start the building option "build roof"
7. Aftzer holding left mouse-button the animation starts
8. White cycle is filling but at the end no roof was builded
After removing all parts and start building again without building a fence next to the watchtower there are no problems to start the roof-building-option.