A catapult would be the perfect thing to have in any medieval thing.
And i like the model.
Make the grey rivets fit the blocko style, make them octagonal like that wheels and maybe add a small weight on the other end of the "catapult"
I don't think a weight added will look much better.
It would make it look more realistic, but i don't think it will make it look any better.
However, i don't know for sure.