Funny thing about that. Someone here in Sweden emailed the police if it was legal to own a trebuchet or if you needed a weapons liscense for it. (The police issue weapon licenses)

The response was that of course no one has ever tried, so there is no formal ruling. But crossbows require a weapons license while bows do not. Because the crossbow can store the energy. So leaning on that reasoning. It would probably require a license.

But I love that the question was asked to our police, and that they gave a serious and thorough answer.

So, technically. As long as you’re not breaking any traffic related laws. It is not illegal to mount a trebuchet on a roadworthy wagon and tow it behind your car.