For me it's so fun to try to break through an enemy city's walls and also to hide behind my own city's walls and have my archers pew pew at the besieging army. It'd be a dream come true for me if at some point defensive city walls were added to tactical battles in an update or expansion. I vaguely remember that MoM had some type of elementary wall system in their tactical battles; nothing fancy, but adds a nice layer of depth to city battles.
