The trouble with the migrants is caused by the merchants who refuse to leave. If they die in the sieges and we survive, we should be able to get migrants and normal traders again. The trolls are gonna be an issue, but if you can keep our marksbolds up off the ground and inside the battlements, they might be able to frighten the invaders off. That's how the sieges I dealt with ended. Several dead greenskins and beakdogs and the rest booking it with piss running down their pant legs.