Another Siege feature
Somebody as already posted this above i think but, ill try to explain in detail.
Another type of siege technique that can be used is the digging under the enemy walls either and plant explosives under them to collapse them or tunnel right under them to get into the city. Now of course this type of attack would only be available to infantry due to the tunneling.
For the defensive side there are many methods of counter attacking this sort of attack. Firstly the defender could have upgrades built in his city such as strategically placing a boundary of heavy rocks around the wall in the ground potentially slowing down the digging process of the enemy.
Secondly the defender could have other upgrade such as ditches(MOATS) around the castle walls, stopping the defender from bringing in siege ladders and towers and even stopping the tunneling attack. Making the attack have to use other siege equipment to attack the walls.
I'm not sure that is the MOATS are in the game but it would be a good idea.