Am I the only one surprised to see trebuchets as a trainable unit? Shouldn't they be siege equipment with build points like a ram or a ladder?

No medieval army was dragging trebuchets around Europe, not even dismantled. They were constructed on site using wood from the immediate area and once constructed they were essentially immobile. Only component parts might have been carried with an army, like some of the metal components. But for all intents and purposes, a treb was a building. Hinged counterweight trebuchets didn't even have wheels.

I can understand why other smaller siege engines are modeled as units, since they can be moved and they can arguably be used against units of men in the field. But a treb is useless against anything but a fixed structure in a siege, they take too long to reload and once constructed have almost no ability to change aim in the left or right direction, only up and down (which was not easily achieved, either).

I can understand if they limited the capability of constructing trebs at a siege to factions that have built a specific building, to model the fact that advanced knowledge was needed to design and build them effectively. And I wouldn't mind this so much if I didn't suspect that the movement points for siege engines will be very low (as it was in RTW), meaning that having to drag these monsters around will significantly slow down your armies on the campaign map.

- DCD