What if we came up with a new siege engine for the Haven? Something like a whaling harpoon. Or replace the ballista with it. For example, one of the heroes could be made a specialist in this "wonder," and if the idea of an external redesign of the machine is technically difficult to implement, then the machine can simply stay as it is, subject to the "Artillery" skill (that's my translation) while adding special combat capabilities. They might look something like this:
1)"Ship Salvo"—the shot has a 20-30% chance to deal damage through creatures' armor;
2)"Boarding Hook"—wounds an enemy creature and sticks in it, resulting in HP loss over several turns, a reduction of 1 morale, or a decrease of 1-2 speed cells for the creature;
3)"Secret Power"—the shot will randomly (20-30%) remove all positive magical effects from the enemy squad