[Daynarr]

AFAIK there is no possibility for now to mod unit bonus to siege or castle defense. That will make it hard to implement siege engines properly since they wont have bonuses in sieges.
Giving large strength for catapult or siege towers might do the trick temporarily as strength is counted in sieges/defenses but attack values should be low. Actually giving high strength and throw boulder ability (like the one Jotun's have) would make a catapult both throw them at large distances and usefull in sieges.