Perhaps some of these ideas could be used to replace the cultural defence of cities, which I always found a little unrealistic.
Definitely. This.
Lets compare two cities in world war two: Stalingrad and Singapore. Stalingrad had its defenses blown to all hell, was bombarded into rubble and partially surrounded. The attackers had armour, heavy artillery, endless waves of aircraft and well-supplied infantry. And it held. The defenders didn't even dream of giving up. They built makeshift defenses, and rebuilt them over and over when they were destroyed. Fought house-to-house, for months on end.
Then we had Singapore, which, in the way of artillery bombardment, only really saw light infantry mortars. Which was at the very limit of the Japanese supply lines. That was the supposedly un-takable fortress, the jewel of British south-east Asia. That could only be assaulted by boats due to being on its own swampy island. And it was taken in a week. Of course, had the soldiers and civilians known what would happen to them after the city was taken, I'm sure they would have fought to the end, using pointed sticks and throwing rocks once the ammo was gone, right down to the last housewife braining soldiers with a rolling pin.
Yes, history buffs, I'm massively oversimplifying things, and the Japanese had taken the water supply, but morale played a huge part in these battles, and not just the morale of the defending units, but of the city itself. Simple culture would not be enough to account for this - no way was an inland industrial city no one had ever heard of more cultural than an ancient trading hub.
Perhaps city morale could function like the current crime/pollution system. More morale gives automatic buildings that improve defenses and bombard defense, and improves the chance for militia units to suddenly appear when an undefended city is attacked, while bad morale causes crime and reduced defense and espionage defense.