It should actually be quite easily manageable to take the settlement with your units. Try advancing one unit of archers so they are just in range of the town center. About half a bowshot back, deploy your second unit of archers. Keep your general near to this second unit. When your first archer unit fires at the defenders in the square (keep them in loose formation if the defender has missile troops, too), some melee unit usually charges out to see them off. Run your first archers away once the enemy meleers get close, while the second archer unit shoots them up. The enemy won't usually run after your skirmishing archers for long, they daren't leave the settlement proper. Instead, they return to the town square... which means you can rinse and repeat until you feel ready to assault the town square. Your general might even find himself in position to charge at the enemy meleers once in a while. You'll probably lose most of one unit of archers if the enemy has missiles, but apart from that you should be able to severely cut your losses if the enemy doesn't have many more troops than you.