You can always do like I did in my Sweboz campaign. It´s a bit like cheating, but hey, who here has got any morals anyway?
I filled Iuvavoaeta with 2 good spies, so I had a 89% chance of opening the gates in case of attack. Then I gathered a small army of just 4 Herunautoz (Germanic Swordsmen) and 1 Speutagardaz (Germanic Pikemen). I waited until Satres and his army was sitting just outside the city, and then I attacked it.
I rushed my Swordsmen through the gates, and attacked the defenders. Meanwhile my pikemen marched around the city, and took up position in a street that Satres army would have to take if they were to reach the city plaza. By the time I had killed the defenders and taken the plaza Satres army had entered the city, but my pikemen held them off for 3 minutes, and so I won the battle. Since retreating rebel armies are destroyed, Satres and all his men dissapeared forever, and I had won a city guarded by over 2500 men using just 5 units and 2 spies.
Of course, this strategy relies on the fact the the garrison of the city is quite weak, otherwise you are screwed. I was lucky, because for some reason the city had only about 300 defenders, and not the usual 1500.