The defenders at the fortress... what should we do with them?

Starhan, I can’t imagine how much you would swear if you were playing a version older than 3.0, because there were exactly twice as many dwarves. In general, dwarves become very strong in the late game, as they manage to upgrade everything they need, get 5th-level runes, and don’t spare resources. Their troops are quite strong and numerous (especially bears). As for sieging a city... I think it’s not worth even trying without powerful magic and/or skilled siege engines. You can bombard the defenders with chaos spells, cast enthrall or berserk, or even blind them in the end. You can also push them back using the Hive spell based on their attack bonus, block their passage with a wall of swords, or even strike them down with divine retribution, although that’s not the best option.