Just a reminder, though, trebuchet (or any other siege weapon,) can't dispose of any units on the offensive anymore. They can damage, and possibly red-line the units, but you'll still need regular forces to mop up the opposition. So, at some point, you will have to risk your landsknect in offensive combat.
If your trebuchets damage the defenders enough you can use your landsknechts for offense with virtually no risk even when attacking cities. Otherwise you can always use the knights of the proposed stack.