For Christmas last year I asked for a wooden trebuchet or catapult kit. While I put it together months ago, I keep forgetting to post photos of it here.
It was a good kit, where you punch out the pre-cut wood pieces and then glue them together following the instructions.
The finished trebuchet actually works (quite well, too). While it's easy for me to understand how a catapult works, the trebuchet is a bit more complicated, so it's nice having a model where I can point to different parts and see what they do in practice.
For me, regardless of how good books and videos are, there's nothing better than being able to hold the real thing and examine it to find out how it works.