Yea. YOu must have a concerted plan before attacking even weak cities.
1. Get your artillery or bowmen together. Get plenty (3-4 units) because you`ll lose some.
2. Get your melee troops (3-4units).
3.Get your arty or bowmen up to the city and get ready for casualties.
4. Attack with artillerry and bust down his defences.
5. When the city defences are less than half at least, go in with your troops- together.
Trick is to have all your armies ready and together before you attack. Then attack like a punching fist. Helps to have a General too.