Why do those improvements disappear? I could understand if it was hardcoded into the game, but it does not happen with civil defense? Why? What special rule does apply to civil defense?

It's bombard defense. If it's greater than 0, the improvement is treated as "obsolete" in city size 2+. Civil Defense has +50% defensive bonus, but no bombard defense, that's why it won't disappear.

Thanks a lot!!!

Thanks a lot!!!

I deleted my Castle due to this problem. Now it will be back.