Defending needs bonus
In moment its far to easy to run over everything with a good stack (city or not).
There needs a mechanism to reflect city defense like walls, etc. I just ran over cities like I do on field armies.
AI should smart enough to stay behind defense and long range attack shouldn't work like just shooting with 1 single range unit every defending unit from the walls.
The game shouldn't work like "create one big stack" and run over everything just because the AI is unable to defend this big stack in any way.