As for fort reduction, *if* there's something like this we need to seriously think under what rules it would go. Right now a limit it to one -1 per province would mean the first guys getting a province gets the option to reduce, but when ownership changes later they don't?
Why limits? If you have the means (gold) AND an army in the province (for enemy provinces), then it should be possible to completly demolish a mighty fort (once you have captured it). Perhaps each level could take one month to complete?
As for forts in your own provinces, the same except possibly it should be abit cheaper and need no army presence.