If I understand the contest, I am to make a 3D object using as many reusable parts as possible.
I am not sure if it has to be functional; but, I can design a Medieval Catapult.
Should be awesome for those warrior scenes.
Some initial ideas:
CC Licensed Image: https://pixabay.com/en/catapult-medieval-rpg-weapon-148509/
And this more complex one:
CC Licensed Image: https://6dsimplemachines.wikispaces.com/homemade+catapults