I pulled off a miracle tonight.
I had just captured one of three dwarven cities with my armies. They had the city surrounded, so I figured we'd be completely safe for a few turns while wounded units heal up. Oh-ho-ho, how wrong I was!
I clicked the End Turn button, and then watched in puzzlement as a small dwarven army approached my forces. Then another one joined up with the first. And then another army arrived. And another. And another. And so on and so forth. Until four full stacks of units, including the dwarven leader and a hero unit, assaulted my new city. I had three less-than-full stacks to fight them with, and many units in said stacks were wounded. The pre-battle screen gives me a verdict of "likely defeat". To make matters worse, one of *my* units was a mission-critical hero. If he dies, and I lose the battle, it's game over.
Just three key factors allowed me to post this story here, rather than in the "how did you die" thread.
First, one of my three stacks was the siege stack. I had four trebuchets and a builder who could repair machinery, and all of them were manning the city walls
Second, most of the enemy force were tier-1 units. They had some nasty surprises, such as hellhounds and musketeers, but for the most part the dwarves were armed with simple axes and crossbows.
And finally, they didn't bring a single siege unit of their own.
While the dwarves were able to keep up a punishing rate of ranged fire each turn, they were unable to engage my own units in melee and bring their numbers to bear until they could smash the gates down. Which they did, eventually. But between my trebuchets raining stones down on their most dangerous units, my goblin untouchables spitting toxic projectiles, and my spellcasting, I was able to keep them at bay for several turns until a great mass of prospectors and crossbow dwarves managed to break into the city. My hero fell after drawing fire for a couple of turns. My builder served as a human shield, keeping angry dwarves from destroying my trebuchets. And when the last dwarf fell, only three trebuchets remained intact. The rest of my units were all killed.
However, it was all worth it! The enemy leader had fallen, and his remaining two cities were left completely unguarded. He had thrown everything he had at me in one final assault, and it had failed. I swiftly captured the cities in question and proceeded to win the scenario.