One thing I tend to do after gunpowder, providing the faction I'm playing can build culverins or basilisk, is to try and destroy troops on the walls. I'll split my artillery into two groups, both shooting at a wall with troops on it. One group targets next to the left tower while the other group targets the wall next to the right tower. If both sections get damaged to the point where they crack, any troops still on the wall are killed. I'll normally do this once or perhaps twice, then breach both damaged sections and destroy the towers. If I can get troops into the city behind them, I try and block their route back to the square. Fighting in the square causes the most casualties of anything other than assaulting the walls directly, so I try to avoid both of these when possible. The last stand fight only happens when I'm completely out of ammo or the clock is ticking down (happens frequently on citadels).