Attacking a city has been made a lot harder in this game then it was in any other installments in this franchise. If you want to take over a city, you'll have round up a sizable army first and the key element here can be the balance between long-range units (archers, catapults, artillery and etc.) and close range units (warriors, knights, tanks and etc.). Long range units have the advantage because they can attack the city without the city attacking them, unlike the close range units who take damage every time they attack the city. Attacking with the cannons and the likes first can greatly reduce the enemy strength and make it easier for the rest of your army to attack without losing too many soldiers as you do so. What you do need to keep in mind is that cities can bombard once every turn, so if they choose to aim for your archers it can get very tricky, that is why you should try to get more then one long range units (preferably three or four), so you can give wounded units a quick break without losing too many men or retreating altogether. You can also make it easier for yourself if you specialize your units in raiding cities and give long range units a bonus for attacking in rough terrain (enemies almost always build their cities in rough terrain).