inner walls/defences in Cities.

Let garrisons that lay inside walled city become separated defence structure (walls inside walls), it would give them additional defence sense and allow layered defences that would be helpful, specially in the late game where cities become so large that sometimes they lost any value for defender. Maybe even some building that gives this inner walls to city centre, and of course all special districts that count ass military also gives this inner walls.

EDIT: Ok i didn't know that garrisons actually give additional defences aside to walls, now i know it and use it, so my request is to make it more graphical, lets players see some kind of different defences in places that got bigger bonuses to fortification or defence