I'm sorry, but I have to disagree...
In the earlier eras, with a single turn taking up to fifty years would provide a large amount of time for the attackers to find an opportunity to get past walls - destroying them with explosives or finding a way to sneak in and open the gates, for example. Not to mention the possibility of using ladders and such to go over walls (this is risky, hence the 50% bonus to defenders) - I really don't think I should have to waste a turn of city construction to build a ladder (what, 1 or 2 shields?) I assume such things are provided to all attacking units as standard.
In more modern times, tanks could easily take the walls out with a shot or two. Even Marines and Infantry would pack some form of demolition weapons (mortars, rocket launchers, C4) that would take the walls out.
I'm all for adding realism and variety to the game, but I think that a simple +50% defence bonus works well enough for city walls.