IME It's always easier to just blast everything off the walls with ranged and have 12 guys or so attack the gate manually, or just go up ladders when enemies are thinned out. You can build trebs on the map to blow up the enemy siege gear before you attack to make it safer to stand around shooting everyone. If it's a really huge fief you can even retreat after a few hundred enemies are killed and attack again, getting rid of any partial damage troops took.