Especially since walls are heavily underused in the game now, as some already pointed.
And that's a shame ! Walls are historical and should be fun to play.
Maybe remove the fortresses and have a "dungeon" building instead (yes, dungeons are the wrong historical period, so use the historical one, but I don't know its name) that can't work as a defense without walls ?
So the actual defensive structure will be much more varied, and tactically interesting ?
And of course change the way soldiers fight on walls, so that they don't die immediately from enemy missile (except from missiles on siege towers, obviously), which is the reason nobody garrisons units on walls as of now.