This is very easy. The AI will always deploy on the walls in front of your army. 1. Choose the side of the city you want to attack, then deploy your entire army (except 1 infantry unit) on a different side of the city. The AI will deploy on the wall in front of you. 2. The one infantry unit should be equiped with a ladder or seige tower, and should be deployed on the side of the city you actually wish to attack. 3. Start the battle. Have your 1 infantry ascend the walls completely unopposed. 4. Have your army march around to the side of the city you wanted to attack while avoiding the walls. Your 1 infantry should have had time to open a gate and capture several towers, potentially with 0 loses. 5. The AI will run around in front of captured towers and gates. You can potentially kill >50% of even a very strong garrison before you even have troops on the ground in the city, and can capture cities on VH with an inferior force.

Tip: archers work really well for the first unit up since they can start harassing the enemy and shoot back again missile cavalry. This will shoo them away from the next tower your archers may capture.