BTW, imagine your Trebuchet pounding this castle from afar OSS! Will look amazing watching your Trebuchet being loaded and fired by the Trebuchet crew, a massive boulder flying across the screen and smashing the castle! Wait until you see the stone towers and walls, they complete the castle so nicely, you can have great defenses around your town.
A couple of ideas for the Trebuchet also would be like the cannon in AC where you can have two different types of shot (cannon ball or grapeshot) for the Trebuchet you could have it fling a very strong iron ball, it could cost a lot to fire but would do more damage, or for a bit of fun the Trebuchet could fling a dead cow or horse like they really did during medieval times, or a dead bloated corpse with black death!
And also on the Trebuchet OSS, I still think that it should created at the Siege Workshop with the crew, it comes out with the crew (like the cannon in AC) and the Trebuchet is packed onto a cart with all the bits, they push this cart on the map to where the user directs and when you want to fire it (when it's in range) the crew have hammers and start to build it. To me it seems the only realistic way, and let's face it, it would look amazing! Just an idea though
One more thing on the Castle, you will like the construction frames I am going to do, will have a treadmill crane in some of the scenes and the scaffolding and ladders look great too!