It's really odd that the trebuchet has always been overlooked by playmobil and lego, yet they make many fantasy siege weapons.
And primitive catapults which would not be used much in the high middle ages that they both seem to prefer, with detailed plate armors.
Trebuchets seem to be the most sensible choice, probably a bit harder to make due to the sling, but much more interesting as well, it has far more reach.